This thesis was stimulated throughout the time of my participation in a research project on Dynamic Macroeconomics, supported by the German Research Foundation (DFG). The starting point was the central question of how to integrate price setting firms in a dynamic disequilibrium model. Almost all recent literature on imperfect competition in macroeconomics applies the objective demand approach by assuming that firms know the true demand curve they are faced with. While this approach can be ap plied in temporary monetary equilibrium models, it proves inadequate for formulating price adjustment...