This document is an overview of how authentication, authorization, and accounting are accomplished. For all API calls, your application needs to be authenticated. When an API accesses a user's private data, your application must also be authorized by the user to access the data. For example, accessing a public Google+ post would not require user authorization, but accessing a user's private calendar would. Also, for quota and billing purposes, all API calls involve accounting. This document summarizes the protocols used by Google APIs and provides links to more information.
Access typesRead more in the Authentication and authorization section in the Get Started page.
Using API keys
More information and examples for API keys are provided on the API Keys page.
Using OAuth 2.0
More information and examples for OAuth 2.0 are provided on the OAuth 2.0 page.