Epidemiology is the study of the occurrence of disease in the human population. The two main purposes of epidemiologic studies are (1) to describe the frequency and distribution of disease, and (2) to identify risk factors responsible for disease. The first function assists our planning for health care and determines health priorities, while the second helps to guide treatment and ultimately lead to preventive strategies for disease. The knowledge on the epidemiology of Alzheimer’s disease (AD) and dementia has advanced considerably in the past decade. In this chapter, the epidemiology of AD and dementia will be described with the liberal use of graphs. Non-modifiable risk factors of AD and dementia, namely age, gender, education, and genetic factors, will also be discussed here. Modifiable risk factors such as blood pressure, cholesterol level, smoking, and others will be examined in detail in a subsequent chapter on prevention of AD (Chapter 6).