In its second expanded and updated edition, this textbook covers the theory and applications of dynamical processes whose time evolution is not continuous, but rather is modeled in discrete time steps through difference equations. These discrete models play an increasingly important role - reinforced by the use of computers - in science, engineering and economics. This new edition also covers special applications in financial mathematics. Following a basic introduction with examples, the first part provides a thorough survey of linear systems and their stability properties.The second part deals with nonlinear systems, especially their stability, and includes an excursus on chaos and fractals and an introduction into the more recent theory of positive dynamical systems along with applications in biology and economics.