Sustainability of enterprise Information Systems (ISs) largely depends on the quality of their evolution process and the ability of the IS evolution steering officers to deal with complex IS evolution situations. Inspired by Olivé [1] who promotes conceptual schema-centric IS development, we argue that conceptual models should also be the centre of IS evolution steering. For this purpose we have developed a conceptual framework for IS evolution steering that contains several interrelated models. In this paper we present a part of this framework dedicated to the operationalization of IS evolution – the evolution metamodel. This metamodel is composed of two interrelated views, namely structural and lifecycle, that allow to define respectively the structure of a particular IS evolution and its behaviour at different levels of granularity.