In order to remain competitive, the integration of their information systems is an imperative for many large organisations. Applications that originally have been developed independently are now required to interoperate to support new or different functions of the enterprise. Although the mechanisms for application interoperation exist provided by the technology, due to the sheer number and complexity of the running systems, integration solutions � centralised or distributed�appropriate at the local level do not translate successfully to the whole enterprise. Centralised integration approaches often satisfy only some of the integration requirements, they are very expensive, and are fraught with danger since they imply an �all or nothing� approach. Distributed approaches, on the other hand, suffer from complexity and scalability problems as the number of system interfaces to be implemented and the number of execution-time invocations grows with the number of component applications. This dissertation makes a contribution to the field of Enterprise Application Integration (EAI) within the framework of distributed systems technology. Based on real-life case studies experience, we present here a federated approach that controls the size and complexity of the integration effort by reusing existing systems as much as possible and reducing the number of interacting applications. Only selected local elements are exposed to the organisational milieu, and a consistent supporting infrastructure is provided to make systems interactions possible. Our approach provides a flexible and scalable strategy to enterprise integration, avoiding the shortcomings of traditional approaches. We respect existing organisational structures, and demonstrate how appropriate federation infrastructure and protocols enable the interoperation of existing systems. The three main facets of enterprise knowledge are systematically incorporated into the integration effort: a) by the use of domain ontologies to support data integration; b) by the development of a methodology to include business rules; and c) by the development of FEW, a federated workflow model to implement the business processes of the organisation.