This chapter comments on the papers of Levitt and List and of Camerer. It explains why for most laboratory studies it is only relevant whether the qualitative or directional results of the study are externally valid. It argues that laboratory studies are conducted to identify general principles of behavior and therefore promise to generalize. It then examines whether laboratory experiments live up to this promise. It discusses the extent to which qualitative results persist outside of the lab and how we should respond when they do not. The chapter concludes by arguing that the lab and field methodologies are highly complementary and that both provide important insights to the understanding of economics.