The objective of this work has been to study how to support emergency management training using computer simulations. The work has focused on team decision making and the training of situation assessment in a tactical reasoning process. The underlying assumption is that computer simulations in decision-making training systems should contain pedagogical strategies. Our investigations started with empirical studies of an existing system for training infantry battalion staffs. In order to promote controlled studies in the area, we developed a microworld simulation system, C3Fire. By using a microworld, we can model important characteristics of the real world and create a small and well-controlled simulation system that retains these characteristics. With a microworld training system, we can create similar cognitive tasks to those people normally encounter in real-life systems. Our experimental use of C3Fire focuses on the problem of generating an information flow that will support training in situation assessment. Generated messages should contain information about the simulated world that will build up the trainees' mental pictures of the encountered situations. The behaviour of the C3Fire microworld was examined in an experimental study with 15 groups of subjects. The aim of the system evaluation of C3Fire was mainly to study the information flow from the computer simulation through the training organisation, involving role-playing training assistants, to the trained staff. The training domain, which is the co-ordination of forest fire fighting units, has been chosen to demonstrate principles rather than for its own sake.