The theory of junction circulators is either given in terms of a linear combination of the properties of single-port reactive eigennetworks or in terms of a single-port approximate equivalent circuit. The purpose of this paper is to present a unified theory of circulators using either approach in terms of ABCD matrices, ideal transformers, and resonator eigennetworks. It is found that all standard circulators can be conveniently described in the notation introduced in this text.