A circulant matrix is one in which a basic row of numbers is repeated again and again, but with a shift in position. Such matrices have connection to problems in physics, signal and image processing, probability, statistics, numerical analysis, algebraic coding theory, and many other areas. At the same time, the theory of circulants is easy, relative to the general theory of matrices. Practically every matrix-theoretic question for circulants may be resolved in closed form. Consequently, circulant matrices constitute a nontrivial but simple set of objects that the reader may use to practice, and ultimately deepen, a knowledge of matrix theory. They can also be viewed as special instances of structured or patterned matrices. This book serves as a general reference on circulants, as well as provides alternate or supplemental material for intermediate courses on matrix theory. There is some general discussion of matrices: block matrices, Kronecker products, decomposition theorems, generalised inverses. These topics were chosen because of their application to circulants and because they are not always found in books on linear algebra. More than 200 problems of varying difficulty are included.