Plasticity Theory is a basic tool of structural analysis used to evaluate ultimate strength and post-elastic behaviour of ductile structures. Modern computing codes make use of it, often by exploiting sophisticated computational analysis and advanced kinematics for large deformation. Yet all this valuable effort has its Achilles heel: the evolution law of the yield surface, also referred to as the work-hardening rule. This book approaches the work-hardening problem in a new and deductive way. It starts with a few elementary, hardly questionable facts and works out their inescapable consequence, following them through until a practical solution is obtained. The analysis it presents will show that plastic yielding of ductile metals is essentially virgin yielding - no matter how strong the material was cold worked originally.