Autocatalytic interactions between the members of an animal group or society, and particularly chemically or visually mediated allelomimesis, can be an important factor in the organisation of their collective activity. Furthermore, the interactions between the individuals and the environment allow different collective patterns and decisions to appear under different conditions, with the same individual behaviour. While most clearly demonstrable in social insects, these principles are fundamental to schools of fishes, flocks of birds, groups of mammals, and many other social aggregates. The analysis of collective behaviour in these terms implies detailed observation of both individual and collective behaviour, combined with mathematical modelling to link the two.