The human body is under various continuous external conditions, some of which make wearers uncomfortable. Usually, human thermal comfort depends on combinations of clothing, climate, and physical activity. Specifically thermal insulation in clothing is an important parameter of thermal comfort. Therefore, this paper discusses theoretically the basic physical principles of the body mechanism for heat transfer with the environment. In this study, the body heat balance was examined, and the effects of clothes and various climatic conditions on thermal comfort were investigated for different physical activities.