The problem of computing the deformations of viscoelastic, viscoplastic or elastoplastic bodies is of great relevance to practical problems e.g. in the numerical modelling of viscoelastic structures of hot metal forming or of polymer melt flows. This book shows how such practical engineering problems may be written as standard problems of functional analysis and how this mathematical framework is used for their analysis and numerical solution. The author looks at the mechanical and the mathematical problem in a unified context, especially for the case of nonlinear Maxwell problems, and gives a mixed discrete formulation for viscoelastic flow problems. Some classical numerical strategies are described as extensions of the mathematical theory. The treatment is restricted to small strain situations (with an introduction to the large strain problem at the end) and to constitute laws of differential type, and will be of interest to researchers and graduate students in mechanical engineering, computer science and applied mathematics.