The title of this book is derived from a graduate course in which Professor Egon Orowan presented to M.I.T. students a clear and simple picture of the basic concepts in crystal plasticity and the mechanics of fracture of materials. Since the publication of his pioneering papers on dislocations and atomic mechanisms of fracture in the early 1930's,Professor Orowan has been one of the principal contributors to the field of physics of plasticity and strength. During the past 10 to 15 years, the perfection of many direct experimental methods has caused a great increase of activity in the elucidation of the effects of dislocations on mechanical and physical properties. Equally intensive activity is taking place in the field of physics and mechanics of fracture processes. Professor Ali Argon felt that the retirement of Professor Orowan from the M.I.T. faculty was an appropriate occasion to take stock of the developments in the immediate past and to produce a needed synthesis of this technologically important field. For this purpose he invited 37 of the world's leading figures in the field to contribute theoretical papers of original work. The 17 papers on the Physics of Plasticity fall into two categories: (1) Individual Dislocations and Basic Deformation Mechanisms, and (2) Hardening Mechanisms and Dislocation Dynamics. The 10 papers on the Physics of Strength concentrate on (1) Cracks and Fracture, and (2) Geology.