A continuation of The Theory of Matroids, (edited by N. White), this volume consists of a series of related surveys by top authorities on coordinatizations, matching theory, transversal and simplicial matroids, and studies of important matroid variants. An entire chapter is devoted to matroids in combinatorial optimization, a topic of current interest. Care has been taken to ensure a uniform style throughout, and to make a work that can be used as a reference or as a graduate textbook. Excercises are included.