Epidemiology is the study of distributions and determinants of health related states or events in a specified population and application of this study to control those health problems. Epidemiologic studies can be broadly classified as OBSERVATIONAL STUDIES AND EXPERIMENTAL STUDIES. Observational studies can be descriptive epidemiological studies or analytical studies. Descriptive epidemiology includes describing the diseases or health related states or events in terms of time, place and person. When it is occurring?, Where it is occurring and who all are affected? Analytical epidemiological studies are used to study the relationship between exposure to a risk factor and developement of disease or health outcome. Analytical studies can be either CASE CONTROL STUDY or COHORT STUDY.