Coordination failures constitute an alternative explanation for underemployment that complements the Keynesian and neo-classical views. The paper proposes to distinguish three classes of models with coordination failures. The classes are formed by strategic models with or without a coordinating role for prices, and general equilibrium models. The main insights resulting for each class of models are exhibited. It is argued that coordination failures are likely to arise in a decentralized economy, even under conditions where perfect competition could prevail. The paper concludes by pointing out several promising directions for future research.