The fundamentals of solar radiation are presented here. Irradiance and irradiation are defined; we explain the origin of the energy emitted by the sun and reaching the ground and its amount as a function of the wavelength – the spectral distribution. The energy reaching the earth depends on the geometry of the earth relative to the Sun. This geometry is described as well as its variation throughout the year. The concept of time is very important in solar radiation. It is detailed here and the notions of mean solar time and true solar time are dealt with. The apparent course of the sun in the sky is described; the zenithal, elevation and azimuth angles are defined. We offer a series of equations to compute the radiation at the top of the atmosphere – the extraterrestrial irradiation- for any instant and for any inclined surface. During its path downwards to the ground, the constituents of the atmosphere deplete the incident light. We introduce the concepts of scattering a! nd absorption. We discuss the main processes affecting the incident radiation in clear and cloudy atmospheres, and especially the effects of molecules, aerosols, gases and clouds. Several examples are given that illustrate atmospheric effects as a function of the solar zenithal angle and atmospheric optical properties. The spectral distribution of the irradiance is discussed for several different conditions. The direct, diffuse and reflected components of the irradiance are defined. How to compute them on an inclined surface is briefly discussed. Many equations are given in this contribution that can be easily introduced in e.g., a spreadsheet or a computer routine, to reproduce the figures.