This book presents a description of the collective behavior of dislocation ensembles based on a continuous description using the powerful tools of the mathematical theory of partial differential equations. Because, in contrast with conventional theories of the plasticity, it features the relevant interaction mechanisms between the participating crystal defects, the theory is able to describe the emergence of dislocation structures. Examples show the intermittent and self-organized character of plastic activity, the effects of sample size on the plastic behavior, and the impact of complex loading paths on the material work hardening.