This chapter describes epidemiology of gingival and periodontal diseases. Epidemiology is the study of health and disease in populations, how the states are influenced by heredity, biological, physical and social environmental ways of living. This chapter covers the types and aims of epidemiology, design of a randomized clinical trial (RCT), definition, criteria for selection, uses and characteristics of an index, and various indices used to study periodontal problems. Experimental epidemiology is used to test hypothesis further by introducing a preventive or therapeutic agent and comparing the outcome in the test subjects, with concurrent observations in control groups. Indices are numerical values describing the relative status of the population on a graduated scale with definite upper and lower limits, which are designed to permit and facilitate comparisons with other populations and are classified by the same criteria and methods.