The origin of the word epidemiology is from the Greek word ‘epi’ meaning upon, ‘demos’ meaning people and logos meaning ‘doctrine’; the literal translation would be ‘the doctrine of what is upon the people’. The primary unit of concern is groups of person not individuals. The most important use of epidemiology is to increase the understanding of disease, shared with the other medical sciences, but looking at communities or populations. For this purpose, epidemiology seeks to determine the cause of disease by studying the disturbances in the equilibrium between agent, host and environment factors, and by identifying the risk factors. Measuring health by morbidity and mortality statistics, and measuring disease frequency in terms of incidence and prevalence is required for obtaining necessary for health planning and development and management of programs for disease prevention and control. Epidemiological studies are required to measure the rates of disease occurrence and the associated factors in a population, to make an unbiased comparison of those with or without a disease or risk factor and to make interventions. This is achieved by a good research design. The design of an epidemiological study serves the function of a measuring instrument. The reader can get a clear understanding of epidemiological study designs through this chapter, including descriptive, analytical and experimental studies. In addition, a section of this chapter provides information on screening of disease. The essence of epidemiology is to determine the causation of disease, i.e. to find out the specific cause or causes of the disease and to assist in its prevention and control, the concepts of association and causation are dealt with in the last segment of the chapter. Overall, this chapter serves as a comprehensive guide to basic epidemiological principles and methods.