This commentary examines the scope of epidemiology and delineates the role of epidemiology in relation to public health. Epidemiology is a science; public health is a mission that is implemented through societal action. The implications of this difference are considered, and the sufficiency of epidemiology for guiding public health is evaluated in relation to other scientific disciplines and nonscientific considerations. The authors conclude that epidemiology is not the basic science of public health but one of many contributors to guiding action. The need for public health decisions despite scientific uncertainty and the potential for epidemiologic certainty's failing to provide clear guidance to public health action emphasize the distinctiveness of these endeavors. Criticisms that epidemiology fails to solve major public health problems, such as tobacco use; that it overemphasizes methods; that it fails to meet the needs of public health practitioners; and that it isolates itself from public health ethics are unwarranted. However, epidemiology should focus on addressing issues that directly affect public health decisions and should clearly communicate information about such issues to public health workers. Public health is far more complex than merely applying epidemiology.