Dental caries afflicts humans of all ages and in all regions of the world. It is a disease that may never be eradicated because of complex interplay of social, behavioral, cultural, dietary and biological risk factors that are associated with its initiation and progression. This chapter describes the pattern of global distribution of caries, the factors responsible for the changing trends of caries in industrialized and developing countries. A basic understanding of the mechanisms of the caries process and histopathology of dental caries is explained through this chapter. Factors affecting the epidemiology of dental caries were depicted as three circles representing the three prerequisites for dental caries (Keyes Triad). Three indispensable factors for development of caries were carbohydrate (diet), bacteria (dental plaque), and susceptible teeth (the host) (Keyes and Jordan, 1963). Since then, many modifying factors have been recognized, resulting in a more complex model that includes saliva, the immune system, time, socioeconomic status, and level of education, lifestyle behaviours, and the use of fluorides. The chapter also highlights the development and epidemiology of early childhood caries in children and root caries that is often seen in the elderly.