An observational study is an empiric investigation of the effects caused by a treatment, policy, or intervention in which it is not possible to assign subjects at random to treatment or control, as would be done in a controlled experiment. Observational studies are common in most fields that study the effects of treatments on people.The second edition of Observational Studies is about 50% longer than the first edition, with many new examples and methods. There are new chapters on nonadditive models for treatment effects (5) and planning observational studies , and the chapter on coherence (9) has been extensively rewritten.Paul R. Rosenbaum is Robert G. Putzel Professor, Department of Statistics, The Wharton School of the University of Pennsylvania. He is a fellow of the American Statistical Association.