Pedagogical in style, this book provides insights into plasma behavior valid over twenty orders of magnitude in both time and space. The book assumes that the reader has a basic knowledge of magnetohydrodynamics and explains topics using detailed theoretical analysis supported by discussion of relevant experiments. This comprehensive approach gives the reader an understanding of the essential theoretical ideas and their application to real situations.The book starts by explaining the topological concept of magnetic helicity and then develops a helicity-based model that predicts the ultimate state towards which magnetically-dominated plasmas evolve. The model predicts that no matter how messy or complicated the dynamics, a great range of plasma configurations always self-organize to a unique, simple final state. This self-organization, called relaxation, is a fundamental concept that unifies understanding of spheromaks, solar corona loops, interplanetary magnetic clouds, and astrophysical jets.After establishing why relaxation occurs, the book then examines how relaxation occurs. It shows that relaxation involves a sequence of complex non-equilibrium dynamics including fast self-collimated plasma jets, kink instabilities, magnetic reconnection, and phenomena outside the realm of magnetohydrodynamics.