Zwicky (1985) has argued that an adequate realizational theory of morphology must incorporate RULES OF REFERRAL in order to account for some kinds of inflectional syncretism. In this paper I propose a formal theory of rules of referral within the broader framework of Paradigm Function Morphology. This theory affords a precise account of a range of rule interactions involving rules of referral; in addition, it furnishes a simple explanation for the fact that syncretisms don't always encompass whole words, for the fact that some referrals are bidirectional, and for the fact that two or more referrals may participate successively or simultaneously in the definition of a single instance of syncretism.