The first part of the chapter explains about Properties of thermal radiation, Prevost’s theory, Heat transfer, Physical effects of heat on matter (Expansion, acceleration of chemical action, thermionic emission). The second part explains about Temperature and measurements, Scales of temperature (centigrade scale, absolute scale, and Fahrenheit scale), types of thermometer (Mercury thermometer, Clinical thermometer, Thermister, Thermocouple). The last part explains about Relative humidity, Specific heat, thermal capacity, Change of state, Regulation of body temperature, Physiological effect of heat (Metabolism, Blood flow, Indirect effects), Methods of heating the tissues (infrared radiation, Microwave diathermy, Paraffin wax, short wave diathermy) and Application in nursing.