This is the first unified treatment of the properties of thermodynamically open and closed systems. It provides the theory and methodology that are necessary to understand nonlinear processes. The section on Classical Systems covers topics ranging from the evolution of probability to open and closed systems and non-Hamiltonian systems. The concluding section on Quantum Systems is equally detailed, treating the evolution of quantum systems, c-number fluctuations and operator fluctuations. The material covered is applicable to weather systems, ocean currents, dye lasers and many other nonequilibrium systems. The text is also suitable for students in graduate course. Numerous physical chemical examples facilitate self-study.