Various events have led to the development of highly complex cooperative operations and to concepts for understanding operations. However. development of membership structures and concepts for understanding these structures has lagged. This paper imports organizational design and contingency theory into the member control literature. Membership structure is understood as organization-like, producing a service (Le., member control). Member control structure is understood as having three aspects (representation, policy making, and oversight) and two environments (the members themselves, and management and operations). Building from cooperative principles and following the development of cooperatives from simple to complex organizations, this paper develops a series of axiomatic propositions for understanding and designing membership structure. Only some of the propositions are testable, and still others are meant only to give continuity and relevance to the propositions as a group (as a theory). Such work should help develop a language for understanding and furthering discussion and research of membership structure and member control in agricultural cooperatives.