Environmental monitoring and enforcement are controversial and incompletely understood. This survey reviews what we do and do not know about the overall effectiveness, as well as the cost effectiveness, of pollution monitoring and enforcement. We ask five key questions: what do environmental monitoring and enforcement actionslook like in the real world? How do we assess environmental compliance and deterrence? Do environmental monitoring and enforcement actions get results? How, why, and when do inspections and sanctions achieve compliance and reduce pollution? And, what do the answers to the preceding questions tell us about designing and implementing more effective and more cost effective public policies for the environment? A key contribution is drawing lessons from diverse sources, including insights from theoretical, empirical, and experimental contributions in environmental, tax, and safety settings. We conclude that traditional environmental monitoring and enforcement actions generate important deterrence effects. However, there are limits to such deterrence, and deterrence itself cannot fully explain all patterns of environmental behavior. Encouraging compliance requires both traditional tools and additional tools.