The book presents a self-contained exposition of the theory of recursion operators, which are closely related to hidden symmetries and Lie-Bäcklund transformations of integrable equations. They allow one to construct infinite hierarchies of integrable models, starting with a simple classical integrable equation, such as the Korteweg - de Vries, Burgers, nonlinear Schrödinger, or massive Thirring equations. In the book, the theory is presented from an abstract geometric point of view. Applications to the above-mentioned classical equations are included too. Recursion operators and hierarchies for supersymmetric generalizations of the classical integrable equations (i.e., mixed Boson-Fermion models related to them) are also presented in a detailed form.