# Adding entities to services

Copy as Markdown[View as Markdown](/docs/development/guides/services/adding-to-services/entities.md)

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What are entities?

Entities are a "**optional**" resource in EventCatalog. You can learn more reading our [entities guide](/docs/development/guides/domains/entities/adding-entities.md).

**Added in** `eventcatalog@2.36.0`

Once you have [created your entities](/docs/development/guides/domains/entities/adding-entities.md) you can add them to your services.

To add an entity to a service you need to add the entity to the `entities` array in the service's markdown file.

/services/PaymentService/index.mdx

```

---
id: PaymentService
name: PaymentService
version: 1.0.0
# This assumes you have an entity called Payment
entities:
  - id: Payment
    # Optional, if not provided the latest version will be used
    version: 1.0.0
---

Your service markdown...
```

## Visualize entity relationships[​](#visualize-entity-relationships "Direct link to Visualize entity relationships")

**Added in** `eventcatalog@3.25.1`

When a service has entities assigned, EventCatalog generates a dedicated **Entity Map** page in the visualizer at `/visualiser/services/{id}/{version}/entity-map`. A link to this view appears automatically in the service sidebar under the visualizer section.

The entity map shows each entity as a node and draws edges between entities that reference one another using the `references` field in their properties. This gives you a quick overview of how the domain model is structured inside a service.

To learn how to define relationships between entities, see the [entity map guide](/docs/development/guides/domains/entities/domain-entity-map.md).