This paper develops a formal model for exchange within a system of action. The system of action is defined by actors, events, control of actors over events, and interests of actors in the outcomes of events. The system is designed to deal with divisible events (best exemplified by private goods) or indivisible events (such as a bill on which a vote is taken), and events with or without externalities. This paper considers mostly divisible events, but some steps are taken toward analysis of indivisible events on which there are opposing interests.