This book studies the implications of macroeconomic complementarities for aggregate behavior. The presentation is intended to introduce Ph.D. students into this sub-field of macroeconomics and to serve as a reference for more advanced scholars. The initial sections of the book cover the basic framework of complementarities and provide a discussion of the experimental evidence on the outcome of coordination games. The subsequent sections of the book investigate applications of these ideas for macroeconomics. The topics Professor Cooper explores include: economies with production complementarities, search models, imperfectly competitive product markets, models of timing and delay and the role of government in resolving and creating coordination problems.