diff --git a/frontend/src/pages/entity-configuration.vue b/frontend/src/pages/entity-configuration.vue
index f06e66c..b105dc9 100644
--- a/frontend/src/pages/entity-configuration.vue
+++ b/frontend/src/pages/entity-configuration.vue
@@ -2,13 +2,24 @@
   import { onMounted, reactive, ref } from 'vue'
   import { auth } from '@/auth'
 
-  const fieldTypes = ['TEXT', 'NUMBER', 'BOOLEAN', 'DATE', 'EMAIL', 'PHONE'] as const
+  const fieldTypes = ['TEXT', 'NUMBER', 'BOOLEAN', 'DATE', 'EMAIL', 'PHONE', 'RELATIONSHIP', 'USER'] as const
   type FieldType = typeof fieldTypes[number]
+  type RelationshipType = 'ONE_TO_ONE' | 'ONE_TO_MANY'
+
+  interface FieldDraft {
+    name: string
+    type: FieldType
+    targetEntityId?: number
+    relationshipType: RelationshipType
+  }
 
   interface EntityField {
     id: number
     name: string
     type: FieldType
+    targetEntityId?: number
+    targetEntityName?: string
+    relationshipType?: RelationshipType
   }
 
   interface EntityDefinition {
@@ -24,10 +35,12 @@
   const newEntityName = ref('')
   const editingEntityId = ref<number>()
   const editingEntityName = ref('')
-  const newFields = reactive<Record<number, { name: string, type: FieldType }>>({})
+  const newFields = reactive<Record<number, FieldDraft>>({})
   const editingFieldId = ref<number>()
   const editingFieldName = ref('')
   const editingFieldType = ref<FieldType>('TEXT')
+  const editingTargetEntityId = ref<number>()
+  const editingRelationshipType = ref<RelationshipType>('ONE_TO_ONE')
 
   function request (url: string, method = 'GET', values?: Record<string, string>) {
     return fetch(url, {
@@ -54,7 +67,7 @@
   }
 
   function ensureNewField (entityId: number) {
-    newFields[entityId] ??= { name: '', type: 'TEXT' }
+    newFields[entityId] ??= { name: '', type: 'TEXT', relationshipType: 'ONE_TO_ONE' }
   }
 
   async function addEntity () {
@@ -101,13 +114,19 @@
     const name = draft.name.trim()
     if (!name) return
     await mutate(async () => {
-      const response = await request(`/api/entity-definitions/${entity.id}/fields`, 'POST', {
+      const values: Record<string, string> = {
         name,
         type: draft.type,
-      })
+      }
+      if (draft.type === 'RELATIONSHIP') {
+        if (!draft.targetEntityId) return
+        values.targetEntityId = String(draft.targetEntityId)
+        values.relationshipType = draft.relationshipType
+      }
+      const response = await request(`/api/entity-definitions/${entity.id}/fields`, 'POST', values)
       if (!response.ok) throw new Error('Field create failed')
       entity.fields.push(await response.json() as EntityField)
-      newFields[entity.id] = { name: '', type: 'TEXT' }
+      newFields[entity.id] = { name: '', type: 'TEXT', relationshipType: 'ONE_TO_ONE' }
     })
   }
 
@@ -115,16 +134,24 @@
     editingFieldId.value = field.id
     editingFieldName.value = field.name
     editingFieldType.value = field.type
+    editingTargetEntityId.value = field.targetEntityId
+    editingRelationshipType.value = field.relationshipType ?? 'ONE_TO_ONE'
   }
 
   async function saveField (entity: EntityDefinition, field: EntityField) {
     const name = editingFieldName.value.trim()
     if (!name) return
     await mutate(async () => {
-      const response = await request(`/api/entity-definitions/${entity.id}/fields/${field.id}`, 'PATCH', {
+      const values: Record<string, string> = {
         name,
         type: editingFieldType.value,
-      })
+      }
+      if (editingFieldType.value === 'RELATIONSHIP') {
+        if (!editingTargetEntityId.value) return
+        values.targetEntityId = String(editingTargetEntityId.value)
+        values.relationshipType = editingRelationshipType.value
+      }
+      const response = await request(`/api/entity-definitions/${entity.id}/fields/${field.id}`, 'PATCH', values)
       if (!response.ok) throw new Error('Field update failed')
       Object.assign(field, await response.json() as EntityField)
       editingFieldId.value = undefined
@@ -152,6 +179,22 @@
     }
   }
 
+  function typeLabel (type: FieldType) {
+    if (type === 'RELATIONSHIP') return 'Relationship'
+    if (type === 'USER') return 'User reference'
+    return type.charAt(0) + type.slice(1).toLowerCase()
+  }
+
+  function relationshipLabel (type?: RelationshipType) {
+    return type === 'ONE_TO_MANY' ? 'One to many' : 'One to one'
+  }
+
+  function targetName (field: EntityField) {
+    return field.targetEntityName
+      ?? entities.value.find(entity => entity.id === field.targetEntityId)?.name
+      ?? 'Unknown entity'
+  }
+
   onMounted(loadEntities)
 </script>
 
@@ -205,15 +248,40 @@
             <label class="sr-only" :for="`edit-type-${field.id}`">Field type</label>
 
             <select :id="`edit-type-${field.id}`" v-model="editingFieldType">
-              <option v-for="type in fieldTypes" :key="type" :value="type">{{ type }}</option>
+              <option v-for="type in fieldTypes" :key="type" :value="type">{{ typeLabel(type) }}</option>
             </select>
 
+            <template v-if="editingFieldType === 'RELATIONSHIP'">
+              <label class="sr-only" :for="`edit-target-${field.id}`">Related entity</label>
+
+              <select :id="`edit-target-${field.id}`" v-model="editingTargetEntityId" required>
+                <option disabled :value="undefined">Related entity</option>
+                <option v-for="target in entities" :key="target.id" :value="target.id">{{ target.name }}</option>
+              </select>
+
+              <label class="sr-only" :for="`edit-relationship-${field.id}`">Relationship type</label>
+
+              <select :id="`edit-relationship-${field.id}`" v-model="editingRelationshipType">
+                <option value="ONE_TO_ONE">One to one</option>
+                <option value="ONE_TO_MANY">One to many</option>
+              </select>
+            </template>
+
             <button class="text-button" type="submit">Save field</button>
             <button class="text-button muted" type="button" @click="editingFieldId = undefined">Cancel</button>
           </form>
 
           <template v-else>
-            <div><strong>{{ field.name }}</strong><span class="type-badge">{{ field.type }}</span></div>
+            <div>
+              <strong>{{ field.name }}</strong>
+              <span class="type-badge">{{ typeLabel(field.type) }}</span>
+
+              <span v-if="field.type === 'RELATIONSHIP'" class="reference-detail">
+                {{ targetName(field) }} · {{ relationshipLabel(field.relationshipType) }}
+              </span>
+
+              <span v-else-if="field.type === 'USER'" class="reference-detail">Built-in User entity</span>
+            </div>
 
             <div class="actions">
               <button class="text-button" type="button" @click="startFieldEdit(field)">Edit field</button>
@@ -231,9 +299,25 @@
           <label class="sr-only" :for="`new-type-${entity.id}`">Field type</label>
 
           <select :id="`new-type-${entity.id}`" v-model="newFields[entity.id].type">
-            <option v-for="type in fieldTypes" :key="type" :value="type">{{ type }}</option>
+            <option v-for="type in fieldTypes" :key="type" :value="type">{{ typeLabel(type) }}</option>
           </select>
 
+          <template v-if="newFields[entity.id].type === 'RELATIONSHIP'">
+            <label class="sr-only" :for="`new-target-${entity.id}`">Related entity</label>
+
+            <select :id="`new-target-${entity.id}`" v-model="newFields[entity.id].targetEntityId" required>
+              <option disabled :value="undefined">Related entity</option>
+              <option v-for="target in entities" :key="target.id" :value="target.id">{{ target.name }}</option>
+            </select>
+
+            <label class="sr-only" :for="`new-relationship-${entity.id}`">Relationship type</label>
+
+            <select :id="`new-relationship-${entity.id}`" v-model="newFields[entity.id].relationshipType">
+              <option value="ONE_TO_ONE">One to one</option>
+              <option value="ONE_TO_MANY">One to many</option>
+            </select>
+          </template>
+
           <button class="secondary-button" :disabled="saving" type="submit">Add field</button>
         </div>
       </form>
@@ -270,6 +354,7 @@
   .field-row:last-child { border-bottom: 0; }
   .field-row strong { margin-right: 0.75rem; }
   .type-badge { padding: 0.2rem 0.55rem; border-radius: 999px; color: var(--v0-primary); background: color-mix(in srgb, var(--v0-primary) 13%, transparent); font-size: 0.7rem; font-weight: 700; }
+  .reference-detail { margin-left: 0.65rem; color: var(--v0-on-surface-variant); font-size: 0.8rem; }
   .empty-fields { color: var(--v0-on-surface-variant); font-size: 0.875rem; }
   .add-field { padding: 1rem 1.25rem 1.25rem; border-top: 1px solid var(--v0-divider); background: color-mix(in srgb, var(--v0-surface-variant) 35%, transparent); }
   .field-inputs input { flex: 1; }
diff --git a/frontend/tests/entity-configuration.spec.ts b/frontend/tests/entity-configuration.spec.ts
index 425a61d..7f94a3c 100644
--- a/frontend/tests/entity-configuration.spec.ts
+++ b/frontend/tests/entity-configuration.spec.ts
@@ -4,6 +4,8 @@ interface EntityField {
   id: number
   name: string
   type: string
+  targetEntityId?: number
+  relationshipType?: string
 }
 
 interface EntityDefinition {
@@ -69,7 +71,13 @@ test('admin can add, edit, and remove an entity and its fields', async ({ page }
       await route.fulfill({ status: 204 })
     } else if (request.method() === 'POST' && segments.at(-1) === 'fields' && entity) {
       const values = formValues(route)
-      const created = { id: nextFieldId++, name: values.name, type: values.type }
+      const created = {
+        id: nextFieldId++,
+        name: values.name,
+        type: values.type,
+        ...(values.targetEntityId ? { targetEntityId: Number(values.targetEntityId) } : {}),
+        ...(values.relationshipType ? { relationshipType: values.relationshipType } : {}),
+      }
       entity.fields.push(created)
       await route.fulfill({ status: 201, json: created })
     } else if (request.method() === 'PATCH' && entity) {
@@ -103,7 +111,7 @@ test('admin can add, edit, and remove an entity and its fields', async ({ page }
   await page.getByLabel('Field type').selectOption('EMAIL')
   await page.getByRole('button', { name: 'Add field' }).click()
   await expect(page.getByText('Website')).toBeVisible()
-  await expect(page.locator('.type-badge')).toHaveText('EMAIL')
+  await expect(page.locator('.type-badge')).toHaveText('Email')
 
   await page.getByRole('button', { name: 'Edit field' }).click()
   const fieldEditForm = page.getByRole('button', { name: 'Save field' }).locator('..')
@@ -111,13 +119,39 @@ test('admin can add, edit, and remove an entity and its fields', async ({ page }
   await fieldEditForm.getByLabel('Field type').selectOption('NUMBER')
   await fieldEditForm.getByRole('button', { name: 'Save field' }).click()
   await expect(page.getByText('Annual revenue')).toBeVisible()
-  await expect(page.locator('.type-badge')).toHaveText('NUMBER')
+  await expect(page.locator('.type-badge')).toHaveText('Number')
 
   page.on('dialog', dialog => dialog.accept())
   await page.getByRole('button', { name: 'Remove field' }).click()
   await expect(page.getByText('Annual revenue')).not.toBeVisible()
 
-  await page.getByRole('button', { name: 'Remove entity' }).click()
+  await page.getByLabel('Entity name', { exact: true }).fill('Contact')
+  await page.getByRole('button', { name: 'Add entity' }).click()
+
+  const organization = page.locator('.entity-card').filter({
+    has: page.getByRole('heading', { name: 'Organization' }),
+  })
+  const organizationFieldForm = organization.getByRole('form', { name: 'Add field to Organization' })
+  await organizationFieldForm.getByLabel('New field').fill('Primary contact')
+  await organizationFieldForm.getByLabel('Field type').selectOption('RELATIONSHIP')
+  await organizationFieldForm.getByLabel('Related entity').selectOption({ label: 'Contact' })
+  await organizationFieldForm.getByLabel('Relationship type').selectOption('ONE_TO_ONE')
+  await organizationFieldForm.getByRole('button', { name: 'Add field' }).click()
+  await expect(organization.locator('.type-badge').filter({ hasText: 'Relationship' })).toBeVisible()
+  await expect(organization.getByText('Contact · One to one')).toBeVisible()
+
+  await organizationFieldForm.getByLabel('New field').fill('Account owner')
+  await organizationFieldForm.getByLabel('Field type').selectOption('USER')
+  await organizationFieldForm.getByRole('button', { name: 'Add field' }).click()
+  await expect(organization.locator('.type-badge').filter({ hasText: 'User reference' })).toBeVisible()
+  await expect(organization.getByText('Built-in User entity')).toBeVisible()
+
+  await organization.getByRole('button', { name: 'Remove entity' }).click()
   await expect(page.getByRole('heading', { name: 'Organization' })).not.toBeVisible()
+
+  const contact = page.locator('.entity-card').filter({
+    has: page.getByRole('heading', { name: 'Contact' }),
+  })
+  await contact.getByRole('button', { name: 'Remove entity' }).click()
   await expect(page.getByText('No entities yet.')).toBeVisible()
 })
diff --git a/src/main/kotlin/Main.kt b/src/main/kotlin/Main.kt
index a0e5bd4..cf226d2 100644
--- a/src/main/kotlin/Main.kt
+++ b/src/main/kotlin/Main.kt
@@ -4,6 +4,7 @@ import dev.mduchene.bolts.entity.EntityDefinition
 import dev.mduchene.bolts.entity.EntityDefinitionRepository
 import dev.mduchene.bolts.entity.EntityField
 import dev.mduchene.bolts.entity.FieldType
+import dev.mduchene.bolts.entity.RelationshipType
 import dev.mduchene.bolts.user.LoginService
 import dev.mduchene.bolts.user.SessionRepository
 import dev.mduchene.bolts.user.UserRepository
@@ -80,11 +81,18 @@ fun main() {
                 val entityId = ctx.pathParam("entityId").toLongOrNull()
                 val name = ctx.requiredFormParam("name")
                 val type = ctx.formParam("type")?.let(FieldType::from)
-                if (entityId == null || name == null || type == null) {
+                val relationship = ctx.relationshipDetails(type, entities)
+                if (entityId == null || name == null || type == null || relationship == null) {
                     if (type == null) ctx.badRequest("A valid field type is required")
                     return@post
                 }
-                val field = entities.createField(entityId, name, type)
+                val field = entities.createField(
+                    entityId,
+                    name,
+                    type,
+                    relationship.targetEntityId,
+                    relationship.type,
+                )
                 if (field == null) ctx.notFound() else {
                     ctx.status(HttpStatus.CREATED).contentType("application/json").result(field.toJson())
                 }
@@ -95,11 +103,19 @@ fun main() {
                 val fieldId = ctx.pathParam("fieldId").toLongOrNull()
                 val name = ctx.requiredFormParam("name")
                 val type = ctx.formParam("type")?.let(FieldType::from)
-                if (entityId == null || fieldId == null || name == null || type == null) {
+                val relationship = ctx.relationshipDetails(type, entities)
+                if (entityId == null || fieldId == null || name == null || type == null || relationship == null) {
                     if (type == null) ctx.badRequest("A valid field type is required")
                     return@patch
                 }
-                val field = entities.updateField(entityId, fieldId, name, type)
+                val field = entities.updateField(
+                    entityId,
+                    fieldId,
+                    name,
+                    type,
+                    relationship.targetEntityId,
+                    relationship.type,
+                )
                 if (field == null) ctx.notFound() else ctx.contentType("application/json").result(field.toJson())
             }
             delete("/api/entity-definitions/{entityId}/fields/{fieldId}") { ctx ->
@@ -138,11 +154,35 @@ private fun io.javalin.http.Context.notFound() {
     status(HttpStatus.NOT_FOUND).contentType("application/json").result("""{"message":"Not found"}""")
 }
 
+private data class RelationshipDetails(
+    val targetEntityId: Long?,
+    val type: RelationshipType?,
+)
+
+private fun io.javalin.http.Context.relationshipDetails(
+    fieldType: FieldType?,
+    entities: EntityDefinitionRepository,
+): RelationshipDetails? {
+    if (fieldType != FieldType.RELATIONSHIP) return RelationshipDetails(null, null)
+
+    val targetEntityId = formParam("targetEntityId")?.toLongOrNull()
+    val relationshipType = formParam("relationshipType")?.let(RelationshipType::from)
+    if (targetEntityId == null || entities.findAll().none { it.id == targetEntityId }) {
+        badRequest("A valid target entity is required")
+        return null
+    }
+    if (relationshipType == null) {
+        badRequest("A valid relationship type is required")
+        return null
+    }
+    return RelationshipDetails(targetEntityId, relationshipType)
+}
+
 private fun List<EntityDefinition>.toJson() = joinToString(prefix = "[", postfix = "]") { it.toJson() }
 private fun EntityDefinition.toJson() =
     """{"id":$id,"name":"${name.toJsonString()}","fields":${fields.joinToString(prefix = "[", postfix = "]") { it.toJson() }}}"""
 private fun EntityField.toJson() =
-    """{"id":$id,"name":"${name.toJsonString()}","type":"$type"}"""
+    """{"id":$id,"name":"${name.toJsonString()}","type":"$type","targetEntityId":${targetEntityId ?: "null"},"targetEntityName":${targetEntityName?.let { "\"${it.toJsonString()}\"" } ?: "null"},"relationshipType":${relationshipType?.let { "\"$it\"" } ?: "null"}}"""
 
 private fun String.toJsonString() = buildString {
     for (character in this@toJsonString) {
diff --git a/src/main/kotlin/dev/mduchene/bolts/entity/EntityDefinition.kt b/src/main/kotlin/dev/mduchene/bolts/entity/EntityDefinition.kt
index 277b1c6..a918ab2 100644
--- a/src/main/kotlin/dev/mduchene/bolts/entity/EntityDefinition.kt
+++ b/src/main/kotlin/dev/mduchene/bolts/entity/EntityDefinition.kt
@@ -4,6 +4,9 @@ data class EntityField(
     val id: Long,
     val name: String,
     val type: FieldType,
+    val targetEntityId: Long? = null,
+    val targetEntityName: String? = null,
+    val relationshipType: RelationshipType? = null,
 )
 
 data class EntityDefinition(
@@ -19,6 +22,8 @@ enum class FieldType {
     DATE,
     EMAIL,
     PHONE,
+    RELATIONSHIP,
+    USER,
     ;
 
     companion object {
@@ -26,3 +31,14 @@ enum class FieldType {
             entries.firstOrNull { it.name.equals(value, ignoreCase = true) }
     }
 }
+
+enum class RelationshipType {
+    ONE_TO_ONE,
+    ONE_TO_MANY,
+    ;
+
+    companion object {
+        fun from(value: String): RelationshipType? =
+            entries.firstOrNull { it.name.equals(value, ignoreCase = true) }
+    }
+}
diff --git a/src/main/kotlin/dev/mduchene/bolts/entity/EntityDefinitionRepository.kt b/src/main/kotlin/dev/mduchene/bolts/entity/EntityDefinitionRepository.kt
index d4fd814..8a5bb51 100644
--- a/src/main/kotlin/dev/mduchene/bolts/entity/EntityDefinitionRepository.kt
+++ b/src/main/kotlin/dev/mduchene/bolts/entity/EntityDefinitionRepository.kt
@@ -6,7 +6,14 @@ import java.sql.ResultSet
 class EntityDefinitionRepository(private val database: Database) {
     fun findAll(): List<EntityDefinition> {
         val fields = database.queryList(
-            "SELECT id, entity_id, name, field_type FROM entity_fields ORDER BY id",
+            """
+                SELECT entity_fields.id, entity_fields.entity_id, entity_fields.name,
+                    entity_fields.field_type, entity_fields.target_entity_id,
+                    entity_fields.relationship_type, targets.name AS target_entity_name
+                FROM entity_fields
+                LEFT JOIN entity_definitions targets ON targets.id = entity_fields.target_entity_id
+                ORDER BY entity_fields.id
+            """.trimIndent(),
         ) {
             FieldRow(
                 entityId = getLong("entity_id"),
@@ -41,30 +48,48 @@ class EntityDefinitionRepository(private val database: Database) {
     fun delete(id: Long): Boolean =
         database.executeUpdate("DELETE FROM entity_definitions WHERE id = ?") { setLong(1, id) } > 0
 
-    fun createField(entityId: Long, name: String, type: FieldType): EntityField? {
+    fun createField(
+        entityId: Long,
+        name: String,
+        type: FieldType,
+        targetEntityId: Long?,
+        relationshipType: RelationshipType?,
+    ): EntityField? {
         val sql = """
-            INSERT INTO entity_fields (entity_id, name, field_type)
-            SELECT id, ?, ? FROM entity_definitions WHERE id = ?
-            RETURNING id, name, field_type
+            INSERT INTO entity_fields (entity_id, name, field_type, target_entity_id, relationship_type)
+            SELECT id, ?, ?, ?, ? FROM entity_definitions WHERE id = ?
+            RETURNING id, name, field_type, target_entity_id, relationship_type
         """.trimIndent()
         return database.queryOne(sql, bind = {
             setString(1, name)
             setString(2, type.name)
-            setLong(3, entityId)
+            setObject(3, targetEntityId)
+            setString(4, relationshipType?.name)
+            setLong(5, entityId)
         }) { toEntityField() }
     }
 
-    fun updateField(entityId: Long, fieldId: Long, name: String, type: FieldType): EntityField? {
+    fun updateField(
+        entityId: Long,
+        fieldId: Long,
+        name: String,
+        type: FieldType,
+        targetEntityId: Long?,
+        relationshipType: RelationshipType?,
+    ): EntityField? {
         val sql = """
-            UPDATE entity_fields SET name = ?, field_type = ?
+            UPDATE entity_fields
+            SET name = ?, field_type = ?, target_entity_id = ?, relationship_type = ?
             WHERE id = ? AND entity_id = ?
-            RETURNING id, name, field_type
+            RETURNING id, name, field_type, target_entity_id, relationship_type
         """.trimIndent()
         return database.queryOne(sql, bind = {
             setString(1, name)
             setString(2, type.name)
-            setLong(3, fieldId)
-            setLong(4, entityId)
+            setObject(3, targetEntityId)
+            setString(4, relationshipType?.name)
+            setLong(5, fieldId)
+            setLong(6, entityId)
         }) { toEntityField() }
     }
 
@@ -76,7 +101,15 @@ class EntityDefinitionRepository(private val database: Database) {
 
     private fun fieldsFor(entityId: Long): List<EntityField> =
         database.queryList(
-            "SELECT id, name, field_type FROM entity_fields WHERE entity_id = ? ORDER BY id",
+            """
+                SELECT entity_fields.id, entity_fields.name, entity_fields.field_type,
+                    entity_fields.target_entity_id, entity_fields.relationship_type,
+                    targets.name AS target_entity_name
+                FROM entity_fields
+                LEFT JOIN entity_definitions targets ON targets.id = entity_fields.target_entity_id
+                WHERE entity_fields.entity_id = ?
+                ORDER BY entity_fields.id
+            """.trimIndent(),
             bind = { setLong(1, entityId) },
         ) { toEntityField() }
 
@@ -87,4 +120,7 @@ private fun ResultSet.toEntityField() = EntityField(
     id = getLong("id"),
     name = getString("name"),
     type = FieldType.valueOf(getString("field_type")),
+    targetEntityId = getLong("target_entity_id").takeUnless { wasNull() },
+    targetEntityName = runCatching { getString("target_entity_name") }.getOrNull(),
+    relationshipType = getString("relationship_type")?.let(RelationshipType::valueOf),
 )
diff --git a/src/main/kotlin/dev/mduchene/bolts/persistence/Database.kt b/src/main/kotlin/dev/mduchene/bolts/persistence/Database.kt
index 17f7911..223cd9c 100644
--- a/src/main/kotlin/dev/mduchene/bolts/persistence/Database.kt
+++ b/src/main/kotlin/dev/mduchene/bolts/persistence/Database.kt
@@ -43,6 +43,9 @@ class Database(private val config: DatabaseConfig) {
                 statement.execute(CREATE_SESSIONS_TABLE)
                 statement.execute(CREATE_ENTITY_DEFINITIONS_TABLE)
                 statement.execute(CREATE_ENTITY_FIELDS_TABLE)
+                statement.execute(ADD_FIELD_TARGET_ENTITY)
+                statement.execute(ADD_FIELD_RELATIONSHIP_TYPE)
+                statement.execute(ADD_FIELD_TARGET_ENTITY_CONSTRAINT)
             }
         }
     }
@@ -99,9 +102,37 @@ class Database(private val config: DatabaseConfig) {
                 entity_id BIGINT NOT NULL REFERENCES entity_definitions(id) ON DELETE CASCADE,
                 name VARCHAR(100) NOT NULL,
                 field_type VARCHAR(30) NOT NULL,
+                target_entity_id BIGINT REFERENCES entity_definitions(id) ON DELETE CASCADE,
+                relationship_type VARCHAR(30),
                 created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP,
                 UNIQUE (entity_id, name)
             )
         """
+
+        const val ADD_FIELD_TARGET_ENTITY = """
+            ALTER TABLE entity_fields
+            ADD COLUMN IF NOT EXISTS target_entity_id BIGINT
+        """
+
+        const val ADD_FIELD_RELATIONSHIP_TYPE = """
+            ALTER TABLE entity_fields
+            ADD COLUMN IF NOT EXISTS relationship_type VARCHAR(30)
+        """
+
+        const val ADD_FIELD_TARGET_ENTITY_CONSTRAINT = """
+            DO ${'$'}$
+            BEGIN
+                IF NOT EXISTS (
+                    SELECT 1 FROM pg_constraint
+                    WHERE conname = 'entity_fields_target_entity_id_fkey'
+                ) THEN
+                    ALTER TABLE entity_fields
+                    ADD CONSTRAINT entity_fields_target_entity_id_fkey
+                    FOREIGN KEY (target_entity_id)
+                    REFERENCES entity_definitions(id)
+                    ON DELETE CASCADE;
+                END IF;
+            END ${'$'}$
+        """
     }
 }
diff --git a/src/test/kotlin/dev/mduchene/bolts/entity/FieldTypeTest.kt b/src/test/kotlin/dev/mduchene/bolts/entity/FieldTypeTest.kt
index b74a84d..8ef3273 100644
--- a/src/test/kotlin/dev/mduchene/bolts/entity/FieldTypeTest.kt
+++ b/src/test/kotlin/dev/mduchene/bolts/entity/FieldTypeTest.kt
@@ -15,4 +15,10 @@ class FieldTypeTest {
     fun `rejects unsupported field types`() {
         assertNull(FieldType.from("attachment"))
     }
+
+    @Test
+    fun `parses relationship cardinality`() {
+        assertEquals(RelationshipType.ONE_TO_ONE, RelationshipType.from("one_to_one"))
+        assertEquals(RelationshipType.ONE_TO_MANY, RelationshipType.from("ONE_TO_MANY"))
+    }
 }