Damage mechanics is concerned with mechanics-based analyses of microstructural events in solids responsible for changes in their response to external loading. The microstructural events can occur as cracks, voids, slipped regions, etc., with a spatial distribution within the volume of a solid. If a solid contains oriented elements in its microsctructure, e.g. fibers, the heterogeneity and asisotropy aspects create situations which form a class of problems worthy of special treatment. This book deals with such treatments with particular emphasis on application to technological composite materials. Chapter one describes the basic principles underlying both the micromechanics approach and the continuum damage mechanics approach. It also reviews the relevant statistical concepts. The next three chapters are devoted to developments of the continuum damage mechanics approach related to characterization of damage with internal variables, evolution of damage and its coupling with other inelastic effects such as plasticity. Chapter 5 describes observations of damage from notches in composite laminates and puts forward some pragmatic modelling ideas for a complex damage configuration. The next two chapters form the bulk of the micromechanics approach in this volume. The first one deals with microcracking and the other with interfacial damage in composite materials.