This book presents all the theoretical and practical basements of heterogeneous kinetics and reactivity of solids. It applies the new concepts of reactivity and spatial function, introduced by the author, for both nucleation and growth processes, with a unified presentation of the reactivity of bulk and powder solids, including gas-solid reactions, thermal decompositions, solid-solid reactions, reactions of solid solutions, and coalescence of solid grains. It also contains many exercises and problems with solutions included, allowing readers to understand and use all the concepts and methods discussed therein.