This paper is an exposition of the theory of regular expressions and its applications to sequential circuits. The results of several authors are presented in a unified manner, pointing out the similarities and differences in the various treatments of the subject. Whenever possible, the terminology and notation of sequential circuit theory are used. The topics presented include: the relation of regular expressions to sequential circuits; algorithms for constructing sequential circuits and state diagrams corresponding to a given regular expression; methods for obtaining a regular expression from a state diagram of a sequential circuit, improper state diagrams, algebraic properties of regular expressions, and applications to codes.