An observational study is an empirical investigation of the effects of treatments, policies or exposures. They differ from experiments in that the investigator cannot control the assignment of treatment to subjects. Scientists across a wide range of disciplines undertake such studies and the aim of this text is to provide a sound statistical account of the principles and methods for the design and analysis of observational studies. Readers are assumed to have a working knowledge of basic probability and statistics, but otherwise the account is reasonably self-contained. Throughout, there are extended discussions of actual observational studies to illustrate the ideas discussed. These are drawn from topics as diverse as smoking and lung cancer, lead in children, nuclear weapons testing, and placement programmes for students. As a result, many researchers involved in observational studies should find this a useful companion to their work.