OBJECTIVES: To provide a primer on the physical characteristics of heat from a biometeorological perspective for those interested in the epidemiology of extreme heat. STUDY DESIGN: A literature search design was used. METHODS: A review of the concepts of heat, heat stress and human heat balance was conducted using Web of Sciences, Scopus and PubMed. RESULTS: Heat, as recognised in the field of human biometeorology, is a complex phenomenon resulting from the synergistic effects of air temperature, humidity and ventilation levels, radiation loads and metabolic activity. Heat should therefore not be conflated with high temperatures. A range of empirical, direct and rational heat stress indices have been developed to assess heat stress. CONCLUSION: The conceptualisation of heat stress is best described with reference to the human heat balance which describes the various avenues for heat gain to and heat loss from the body. Air temperature alone is seldom the reason for heat stress and thus heat-related health effects.