From their site: Use Gastus to keep track of your daily expenses. When you're using the API, it's always through an existing user in Gastus. So when you use the API as "john", you get to see and work with what "john" is allowed to do. Pretty much any URL in Gastus can be viewed in its XML form by adding the .xml extension. So /expenses/4 becomes /expenses/4.xml. You can view, create, update, and delete resources through the API.