This chapter aims to study the fundamentals of thermal radiation. Thermal radiation is the radiation emitted by bodies as a function of the fourth power of their absolute temperature. Planck's black body radiation law is presented, which establishes the emissive power of the black body as a function of temperature and wavelength. The integration of this law for all wavelengths results in the Stefan-Boltzmann law. The properties of thermal radiation are presented: absorptivity, transmissivity, emissivity, and reflectivity. The concept of black cavity radiation is discussed, as well as Kirchhoff's law between emissivity and absorptivity. Next, the approach of gray bodies as radiation models for real bodies is presented. The following section deals with the calculations of thermal radiation flux on surfaces. Finally, the chapter concludes with the presentation of concepts of environmental thermal radiation.