The combinatorial study of finite set systems is a lively area of research unified by the gradual discovery of structural insights and widely applicable proof techniques. This book is the first coherent and up-to-date account of the basic methods and results of this study. Much of the material in the book concerns subsets of a set, but chapters also cover more general partially ordered sets. For example, the Clements-Lindstrom extension of the Kruskal-Katona theorem to multisets is discussed, as is the Greene-Kleitman result concerning k-saturated chain partitions of general partially ordered sets. Connections with Dilworth's theorem, the marriage problem, and probability are presented. Each chapter ends with a collection of exercises for which outline solutions are provided, and there is an extensive bibliography. The work is important for postgraduate students and researchers in discrete mathematics and related subjects.