This document defines epidemiology and describes key epidemiological principles and study methods. It defines epidemiology as the study of disease distribution and determinants in human populations. It describes the steps of defining a disease, measuring its frequency through incidence and prevalence, and examining its distribution based on who, when and where is affected. It also discusses determining disease determinants and outlines three major epidemiological study designs: intervention studies, cohort studies, and case-control studies. Intervention studies most closely mimic experiments but cohort studies are more accurate when experiments cannot be done. Case-control studies are fastest but least accurate.