This chapter analyses basic thermal physics, including definitions of temperature and heat energy. Newton’s Law considers mainly heat exchange by conduction and convection. Modification of Newton’s Law has been made by Dulong & Petit and Josef Stefan. The unit of thermodynamic temperature is the kelvin. The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin. A difference or interval of temperature may be expressed in kelvin or in degree Celsius the numerical value of the temperature difference being the same. Transformation of one form of energy into another is possible, for example, a thermally powered generating station can produce electricity. Such transformation is never totally efficient. Energy release in living tissues is based on the chemical oxidation-reduction reaction. Most bodies are non-transparent to infrared radiation. Therefore, only the most superficial layer of the object contributes to the emission of infrared radiation toward the environment.