The agility with which Enterprise Information Systems (EIS) can be changed is of primordial importance in today's fast paced and competitive market. This depends on the changeability of EIS, which in turn is heavily influenced by the coupling between the services implemented in enterprise value chains, in which EIS need to cooperate. This paper describes the concept of enterprise as a service, a collection of dynamically assembled services with a lifecycle centered on the customers, and addresses EIS interoperability as a means to achieve EIS integration. To help systematizing the various aspects relevant to interoperability, this paper proposes a multidimensional interoperability framework, which includes the following dimensions: Lifecycle (with typical development stages), Concreteness (with various levels of abstraction), Interoperability (based on structural compliance and conformance), and Concerns (to deal with non-functional aspects such as security, quality of service and social and legal issues).