In this paper we argue that the communicative constitution of organizations requires not just one, but four types of messages, or more specifically types of message flow or interaction process. Such a variety of message flows is required because complex organizations require distinct types of relations to four "audiences". They must enunciate and maintain relations to their members through membership negotiation, to themselves as formally controlled entities through self-structuring, to their internal subgroups and processes through activity coordination, and to their colleagues in a society of institutions through institutional positioning. These four sorts of communication are analytically distinct, even though a single message can address more than one constitutive task; we need to recognize that complex organizations exist only in the relatedness of these four types of flow.