Three models of social influence are presented and their principles discussed in depth: social impact theory, the social influence model and group situation theory. A detailed model of social impact is developed and combined with the group situation theory. The integration of these concepts is utilized to reanalyse several classical studies in social facilitation, social inhibition, crowding, social loafing, and helping in an emergency. Finally, in order to emphasize their differences, the three models are used to explain the Asch studies. The combination of a detailed version of social impact theory with group situation theory results in a middle range theory of group behaviour.