eServices are the building blocks for loosely-coupled, distributed applications based on the Service Oriented Architecture (SOA) principles. One of the major benefits they offer is interoperability both between components of service oriented systems and between different systems. Still, the variety and diversity of implementations and interpretations of SOA and the vast amount of emerging standards hinder interoperability. This paper examines interoperability requirements and related issues in the three major eServices categories: Web, Grid and P2P services. Our aim is to provide the basis for a roadmap towards improving interoperability of eServices.