As the basic science of public health, epidemiology has been an important approach in public health and also in clinical practice. Epidemiology is now used together with measurement scales and instruments developed by psychologists to assess the subjective well-being of the target population and to identify psychosocial determinants of diseases and outcomes. As rehabilitation psychologists are becoming more involved in the effort to improve patients' quality of life, epidemiologic data are valuable in guiding the establishment of viable goals for rehabilitation psychology and the health care endeavor more broadly. This chapter provides background information about epidemiology: working definitions, types of epidemiology and epidemiologic studies, measures of disease occurrence and potential sources of errors, how to critically evaluate epidemiologic statistics, and sources of data for use in epidemiology. The practice of epidemiology is also illustrated, along with numerous applications to rehabilitation.