What makes a system comprised of microservices tick like a Swiss clock? The contract. The sum off all the APIs of all the microservices. Now, there are several questions that come to mind. How do we formally define the API, how to keep it backwards compatible, how can we be sure the microservices do not break the contract? We will speak from experience showing the different approaches we took in finding the answers to those questions. We will present you the challenges we faced and how we overcame them in order to have large scale continuous delivery.