This book presents the basic theory of directional derivatives and monotonicity that is required in the study of optimization problems and economic theory. The authors discuss the concepts of generalized directional derivatives and generalized monotonicity. They then explain how to use these concepts in the study of general variational inequality problems in terms of bifunctions in finite dimensions and as related to optimality conditions involving differentiable or directionally differentiable functions. The book also covers such topics as nonsmooth convexity.