Astronomers have long noted that the Earth does not rotate uniformly about an axis fixed in the planet, that both the length-of-day and the direction of the rotation axis vary periodically and irregularly by small amounts. These variations are an immediate consequence of the Earth not being a rigid body. In this book Professor Lambeck discusses the irregular nature of this motion and the geophysical mechanisms responsible for it. A complete analysis of these causes requires a discussion of solid Earth physics, magnetohydrodynamics, oceanography and meteorology. The study of the Earth's rotation is therefore of interest not only to astronomers who wish to explain their observations, but also to many geophysicists who use the astronomers' observations to understand better the Earth's response to a variety of applied forces. The author emphasizes the important contributions made over the last 15 years, this progress being in part a consequence of the overall progress in geophysics and planetary physics and of the developments in space science and technology, which not only require that the Earth's motion be precisely known but which also have provided new and precise methods for monitoring this motion. This book is suitable for geophysicists, astronomers and geodesists who are actively engaged in research as well as for graduate students.