> ## Documentation Index
> Fetch the complete documentation index at: https://developers.calendbook.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Authentication Overview

Our API supports two authentication methods: **Machine-to-Machine (M2M)** and **OAuth2 (User Authorization)**.\
Each method is designed for different use cases, so it’s important to choose the right one for your integration.

## When to use M2M Authentication

Use **M2M authentication** when your application or partner system needs to communicate with our API **without user interaction**.\
This is most common for backend services, cron jobs, or B2B partner integrations where the client itself (not an end-user) is the identity.

Example scenarios:

* Server-to-server data sync
* Automated reporting jobs

## When to use OAuth2 Authentication

Use **OAuth2 authentication** when your application needs to access a user’s data **on their behalf**.\
This method requires the user to explicitly grant consent through a secure OAuth2 flow.

Example scenarios:

* A web or mobile app connecting to a user’s account
* Allowing users to manage their calendar, contacts, or files through your app