Participation of members in cooperatives is essential if they are to remain democratic and responsive, yet there is a surprising lack of theory as to why, and under what circumstances, people participate. This article identifies different types of participation, and considers the possibility of deriving a theoretical model from existing literature. Having reviewed related literatures on political, economic and community participation, it finds a need to go back to basic social psychological theories with which to build up useful theory. Two theoretical models are presented: an individualistic theory from George Homans, and a mutual theory from Pitrim Sorokin. These are elaborated in relation to cooperatives; the individualistic theory is found to be useful where members have an instrumental attitude, but the mutual one is shown to be muchricher in explanatory value, and to point to ways participation can be improved.