Modern defence systems include advanced aircraft, ships, radar, weapons, command and control systems, and most importantly human operators. The main objective of modelling and simulation tools is to allow operational analysts to rapidly specify and evaluate existing and proposed systems and procedures for operating these systems. Such tools are required to model all aspects of defence systems including physical systems and human operators and the reasoning processes that they adopt. Agent-oriented technology is a natural candidate for developing a model of reasoning processes performed by human operators. It allows the operational analyst to work at a high level, formulating cognitive processes, while keeping the detailed computer programming hidden. This premise has led to the development of the Operator-Agent. The base model was completed in June 1996. The model is fully operational and is an integral part of the tools used by operational analysts from the Australian Department of Defence. It has been successfully used for operational analysis and evaluation of multi-billion dollar acquisitions.