This report is the result of a study on middleware as being one of the essential concepts in the construction of the information infrastructure for Network Centric Warfare. The purpose of this report is to elaborate on the different aspects of middleware in information systems, under the assumption that future systems development will be architecture-focused, and that the systems developed should fit into an environment of open, distributed systems. The relation between architectual models and middleware, is presented. Some categories containing middleware with similar characteristics, are described. A wide collection of desirable middleware properties, is also presented.