The problem of cooperation between rational self-interested users in wireless networks is studied under the framework of coalitional game theory. It is shown that the stable coalition structure, i.e., the coalition structure for which users have no incentives to defect, depends upon the apportioning scheme chosen to distribute the cooperative rate gains between coalition members. The interference channel and the multiaccess channel are used as illustrative network models to study the formation of stable receiver coalitions under both a flexible (transferable) and fixed (non-transferable) apportioning scheme. The problem of determining stable coalitions for the case of cooperating transmitters in an interference channel is also discussed.