We study a natural notion of compliance between clients and services in terms of their bpel (abstract) descriptions. The induced preorder shows interesting connections with the must preorder and has normal form representatives that are parallel-free finite-state activities, called contracts. The preorder also admits the notion of least service contract that is compliant with a client contract, called dual contract, and exhibits good precongruence properties when choreographies of Web services are considered. Our framework serves as a foundation of Web service technologies for connecting abstract and concrete service definitions and for service discovery.