The present article provides an overview of epidemiological studies in Japan. The origin of modern epidemiology of Japan can be traced back into the late 19th century. Baron K. Takaki at that time made brilliant epidemiological studies on beriberi and was thus able to eradicate the disease long before vitamin B1 deficiency was identified as the cause of the disease. Epidemiological studies really began to flourish in Japan after the end of World War II. Since the most of infectious diseases have been controlled, epidemiological studies on cancer, heart disease, stroke, and other chronic diseases have become the main target of investigations. It may be cautioned that, among infectious diseases, tuberculosis is still a serious health problem today and HIV infection has become a threatening health issue although the number of AIDS patients reported was still about 1,000 for the whole country in 1995. In contrast to other industrialized countries, heart disease is far less common in Japan, probably reflecting still not-too-rich diet among Japanese. There are a number of unique or unusual epidemiological studies in Japan, including a long-term surveillance of those who were exposed to A-bomb irradiation in 1945. Readers are encouraged to refer to detailed description of each, specific topic presented in this volume. Essential vital statistics are also presented as background information of epidemiological studies in Japan.