This book presents the theory of dynamical systems with nonzero Lyapunov exponents, offering a rigorous mathematical foundation for deterministic chaos - the appearance of "chaotic" motions in pure deterministic dynamical systems. These ideas and methods are used in many areas of mathematics as well as in physics, biology, and engineering. Despite the substantial amount of research on the subject, there have been relatively few attempts to summarize and unify results in a single manuscript. This comprehensive book can be used as a reference or as a supplement to an advanced course on dynamical systems.