Plastic deformation of a metal is possible because of the production and motion of large numbers of dislocations. Dislocations are one dimensional flexible spaghetti-like defects that interact with one another with both long range (1/r) and short range atomic scale forces. As the dislocation density grows, these interactions cause tangling and the formation of partially ordered but very complex 3D structures. As a result, the transport of mobile dislocations through the immobilized structures becomes increasingly difficult (work hardening). Attempts to predict the mechanical behavior on the basis of the underlying dislocation physics have been extensive and only partly successful. New methods, both experimental and theoretical are now producing new understanding, and are leading to hope that more sueful predictions might ultimately be made. After a general introduction to the phenomenology and the main fundamental issues, I will present our work focused on the episodic transport of dislocations in face centered single crystals through the hardened structure.