The recent introduction of many agent-based systems and the increase in agent publications have made the area of agent-based systems a popular area. However, a number of challenges must be met before agent-based system development can claim to be fully mature. One of the most important challenges is the use of a comprehensive design approach whereby the design models fully support agent aspects at the design level. This paper describes a design methodology which allows the development of agent based systems from user requirements. The methodology models the external and internal behaviour of agents. It provides a means of both visualizing the behaviour of systems of agents and defining how the behaviour is achieved. It provides a systematic approach for generating from high-level designs implementable system definitions. Humans, with machine assistance, can manipulate in a systematic way models at one level of abstraction into models at the next lower level.