Harmonic maps are maps between Riemannian or pseudo-Riemannian manifolds which extremize a natural energy integral. They have found many applications, for example, to the theory of minimal and constant mean curvature suface. In physics they arise as the non-linear sigma and chiral models of particle physics. Recently, there has been an explosion of interest in applying the methods to ingrable systems to find and study harmonic maps. Bringing together experts in the field of harmonic maps and integrable systems to give a coherent account of this subject, this book starts with introductory articles, so that the book is self-contained. It should be of interest to graduate students and researchers interested in applying integrable systems to variational problems, and could form the basis of a graduate course.