Epidemiology is defined as the study of all aspects of health at the level of population. Its scope includes the description of how diseases and, more generally, health-related conditions occur in the population, as well as the search for the factors at their origin. These factors could span from the level of the molecule — for example, blood cholesterol — to the level of society — for instance, loss of employment. This broad perspective makes epidemiology at the same time a biomedical and a social science. This chapter introduces the five major areas within epidemiology: descriptive, aetiological, evaluative, health services and clinical epidemiology.