1. A system for use in training armed personnel, the system comprising: a number of autonomous mobile units, the mobile units being arranged to to maintain estimates of their own positions within a pre-defined training area; each mobile unit being arranged to move autonomously about the training area under computer control, the autonomous movements of each mobile unit being at least partially based on the estimates of its own positions; and the mobile units being arranged to perform autonomous actions in the training area to simulate participants in a firearms training exercise; wherein at least one of the mobile units is arranged to autonomously base at least some of its actions at least partially on a sensed behavior of at least one human in the training area.