# API functions

With Realm you can create API functions to add dynamic and interactive content.

Define API endpoints by creating files in the `@api` directory.
These endpoints can interact with other APIs, implement custom authentication, or connect to databases.

## Common use cases

Use API functions to:

- Integrate with external APIs: fetch data from services like GitHub using secret tokens stored in environment variables. For example, retrieve pull requests, issues, or repository statistics to display within a Markdoc tag or generate dynamic documentation content.
- Add custom authentication: inject your app’s own API credential system into the project.
- Integrate a payment gateway: securely process payments by integrating with external payment services, keeping API keys and sensitive data server-side.
- Interact with databases: connect to databases like PostgreSQL or MongoDB to fetch, update, and serve data within your project.


## Reference documentation

To reference the documentation for API functions, see:

- [API functions reference](/docs/realm/customization/api-functions/api-functions-reference) for API functions helper methods and properties.
- [Create API functions](/docs/realm/customization/api-functions/create-api-functions) to learn how to build API functions.