This chapter examines how and why Assemblage Theory matters for planning theory. It explains how Assemblage Theory deals with the critique of collaborative approaches toward rationalist planning. The chapter explores how Assemblage Theory fixes important shortfalls of collaborative approaches. In the rational planning model, the planner is a scientist with an objective outside view who has the task of rational trade-offs between interests on the basis of scientific methods. Communicative or collaborative planning situates the planner in a context that is shaped by the logic of negotiation. This moves attention of planning theory to the planner him/herself, in other words: it focuses on the subject, not the object of planning. Assemblage Theory defines the rules or the basic concepts one must follow when studying or interfering with processes of assembly in order to avoid the essentialism that characterizes naive realism and forms of rationalism.