This book presents a description of the collective behavior of dislocation ensembles based on a continuous description, using the powerful tools of partial differential equations instead of conventional theories of the plasticity. This approach reveals the relevant interaction mechanisms between the participating crystal defects, thereby allowing the theory 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.