Most of the solar system exists in the plasma state (the fourth state of matter). Its subtle nonlinear interaction with the magnetic field can be described by the equations of magnetohydrodynamics (MHD). Over the past few years this important and complex field of research has been actively pursued and increasingly diversely applied to the fields of geophysics, space physics, and astrophysics. It is, for instance, relevant to the study of many dynamic phenomena such as solar flares, and the origins of magnetic fields in the Sun and the Earth. This book examines basic MHD topics, such as equilibria, waves, instabilities, and reconnection, and examines each in the context of different areas that utilize MHD. Many of the world's leading experts have contributed to this volume, which has been edited by two of the key enthusiasts. It is hoped that it will help researchers to appreciate and understand the common threads among the different branches of magnetohydrodynamics.