The Keap API uses a fairly standard implementation of OAuth 2.0 in order to provide authentication to all API endpoints. In the past, the Infusionsoft API has relied on a simple token based system; while those tokens will remain active until some date in the future for the XML-RPC API, any new implementations and all requests to the REST API will be required to use OAuth 2.0. Rather than re-explain OAuth again, it is more useful to provide a series of documents that have already been created and demonstrate the OAuth protocol, how to implement it in your code, how to troubleshoot, and how to ease development. Before that, though, it is important to have the authorization destinations and necessary details.