In this book, we shall consider the kinematics and dynamics of the flows of fluids exhibiting a yield stress. To highlight the principal characteristics of such fluids, the first chapter emphasizes the role played by the yield stress. Next, a careful description of the continuum mechanics behind the constitutive equations for incompressible and compressible viscoplastic fluids is given in Chapters 2-4. In Chapters 5 and 6 analytical solutions to several steady and unsteady flows of Bingham fluids are presented. The subsequent Chapters 7-10 are concerned with the development of variational principles and their numerical solutions, along with perturbation methods which play a significant role in numerical simulations.