The purpose of this book is to present a new approach to fundamental questions concerning the calculus of variations (and more generally to the theory of optimization) based on a systematic analysis of Young measures. Weak lower semicontinuity and relaxation are main areas of concentration in this work. The unified treatment of scalar and vector cases developed here is suitable also for more general situations. Applications to problems in continuum mechanics and nonlinear elasticity are analyzed in some depth. Other problems are included to motivate interest in Young measures and to emphasize the wide range of applicability of these techniques (to, for example, optimal control, optimal design and turbulence). Researchers and graduate students in applied mathematics and mechanics will benefit from this general approach to variational principles.