What should you do when a referee says your paper has an "endogeneity problem" or a seminar participant asks how your empirical model is "identified"? This course will present a set of tools and concepts that should help you produce an intelligent response. There are two main goals. The first is to develop a clear understanding of the conceptual difficulties associated with establishing causality in empirical work. In particular to de-mystify several concepts that are frequently invoked as "problems" or "issues" in applied empirical research, but seldom clearly explained. A good grasp of these concepts will lead to improved research design and a sharper understanding of the relative strengths and weaknesses of various statistical methods. The second goal is to describe some of the practical problems that arise in the application of these methods. The course has special emphasis on testing the key underlying assumptions. This course is designed to complement a traditional two-semester graduate sequence in econometrics.