This book provides a self-contained introduction to magnetohydrodynamics (MHD), with emphasis on nonlinear processes. The book outlines the conventional aspects of MHD theory, magnetostatic equilibrium and linear stability theory. It concentrates on nonlinear theory, starting with the evolution and saturation of individual ideal and resistive instabilities, continuing with a detailed analysis of magnetic reconnection and concluding with a study of the most complex nonlinear behavior, that of MHD turbulence. The last chapters describe three important applications of the theory: disruptive processes in tokomaks, MHD effects in the reversed field pinch, and solar flares. In the presentation the focus is more on physical mechanisms than on special formalisms. The book is essential reading for researchers and graduate students interested in MHD processes both in laboratory and in astrophysical plasmas.