Interoperability of services and systems is an important prerequisite for creating eGovernment applications that support co-operation and best practice sharing. The paper compares several promising approaches aimed at achieving these objectives: interoperability (standard) frameworks, web services, intelligent software agents, and semantic web. These approaches are on various levels of readiness and require different resources. A case study is proposed, involving interoperable digital signatures for cross-border electronic services. Development of such services includes dealing with incompatibility problems on all levels of digital signature infrastructure, making agreements between service stakeholders, and implementing projects involving various services. Use of interoperable digital signatures can best be implemented through the web services approach.
