The incompleteness of financial markets and the existence of finance constraints provide an explanation for the sort of coordination problem that afflicts real-world economies, but is absent from simplistic New Classical models. Providing an understanding of monetary and macroeconomic issues in terms of "imperfections," leading economic theorists--including Hicks, Greenwald, Stiglitz, and Aoki--here examine an important new area of macroeconomic theory--the implications of the finance-constraint approach to monetary theory.