This book studies the effects of imperfect competition between firms on the occurrence of macroeconomic disequilibria and their dynamic evolution. It starts out with a comparison of the concepts of subjective and objective demand and surveys the literature on general equilibrium theory with imperfect competition. The objective demand approach is critized not only on the grounds of its strong rationality requirements and existence problems, but also by the finding that it cannot be applied successfully to characterize determinate rational expectations equilibria in intertemporal macroeconomics. Finally, price setting firms using subjective demand functions are integrated in a dynamic disequilibrium model in order to study monopolistic and oligopolistic price adjustment.