The papers in this symposium demonstrate the theoretical as well as the empirical plausibility of indeterminacy and of sunspots in equilibrium models with infinitely lived agents. They depart from the standard competitive framework because of the presence of externalities or of monopolistic competition. Some focus on business fluctuations while others address long-run growth. The second part of the Introduction presents a general unifying framework to identify the structure that is common to the papers of the symposium. Journal of Economic Literature Classification Numbers: E00, E3, O40.