This chapter, based on analysis of literature, introduces a definition and a layered architecture for government information systems. It argues that the interoperability of information systems is essential in providing integrated government services, but unfortunately there is no consensus on what interoperability is and what constitutes interoperable information systems. By analysing available academic literature and government reports on interoperability, the authors have attempted to address this problem. Based on the study, a comprehensive definition of interoperability is presented here. Further, the authors have identified four constituents of interoperable system and finally, a layered architecture for interoperability of government information systems has been presented. The authors believe that an unambiguous definition and clear idea about constituent of interoperability would remove much confusion in conceptualisation, design, and development of interoperable systems in government.