In an environment of constant change, driven by competition and innovation, a service can rarely remain stable - especially when it de- pends on other services to fulfill its functionality. However, uncontrolled changes can easily break the existing relationships between a service and its environment (its customers and providers). In this paper we present an approach that allows for the controlled evolution of a service by lever- aging the loosely-coupled nature of the SOA paradigm. More specifically, we formalize the notion of contracts between interacting services that en- able their independent evolution and we investigate under which criteria can changes to a contract-bound service, or even to the contract itself, be transparent to the environment of the service. Keywords: service evolution, service contracts, compatibility, contract invariance, contract evolution