As electronic Government is increasing its momentum internationally, there is a growing need for the systematic management of the newly defined and constantly transforming services. eGovernment Interoperability Frameworks usually cater for the technical standards of eGovernment systems interconnection, but do not address service composition and use by citizens, businesses or other administrations. An Interoperability Registry is a system devoted to the formal description, composition and publishing of traditional or electronic services, together with the relevant document and process descriptions in an integrated schema. Through such a repository, the discovery of services by users or systems can be automated, resulting in an important tool for managing eGovernment transformation towards achieving interoperability. The paper goes beyond the methodology and tools used for developing such a system for the Greek Government, to population with services and documents, application and extraction of useful conclusions for electronic Government transformation at global level.