The sun produces a vast amount of energy. The energy emitted by the sun is called solar energy or solar radiation. Despite the considerable distance between the sun and the earth, the amount of solar energy reaching the earth is substantial. At any one time, the earth intercepts approximately 180 10⁶ GW. Solar radiation is the earth primary natural source of energy and by a long way. Other sources are: the geothermal heat flux generated by the earth interior, natural terrestrial radioactivity, and cosmic radiation, which are all negligible relative to solar radiation. As a consequence, the solar radiation influences many aspects of the earth, including weather and climate, ocean, life on earth, agronomy and horticulture, forestry, ecology, oenology, energy, architecture and building engineering, or materials weathering. These lecture notes intend to present the fundamentals in solar radiation at earth surface to a wide community. Its content originates from lectures given to students of master degree level or higher, engineers and researchers in climate, geophysics and environment sciences, life sciences, or energy. This document should be valuable to any engineer, scientist and practitioner. The solar radiation received at a given geographical site varies in time: between day-night due to the earth rotation and between seasons because of the earth orbit. At a given time it also varies in space, because of the changes in the obliquity of the solar rays with longitude and latitude. Notwithstanding the effects of the clouds and other atmospheric constituents, the solar radiation received at a given location and time depends upon the relative position of the sun and the earth. This is why both sun-earth geometry and time play an important role in the amount of solar radiation received at earth surface. A major part of this textbook is devoted to this matter. The geometry of the earth relative to the sun 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 solar zenithal, elevation and azimuthal angles are defined. These angles are identical at top of the atmosphere and earth surface; no change is introduced by the atmosphere. Equations are given in this part that can be easily introduced in e.g., a spreadsheet or a computer routine, to compute all quantities and reproduce the figures. Both horizontal and inclined surfaces are dealt with. The amount of solar radiation that is intercepted by the earth varies because of variations in sun-earth distance and as far as the spectral distribution is concerned by day-to-day variations due to solar activity. The closer to the sun the earth, the greater the solar irradiance impinging on a plane normal to the sun rays and located at the top of the atmosphere. The total solar irradiance, often abbreviated in TSI, is the yearly average of this irradiance during a year integrated over the whole spectrum. The variations within a year amount to ± 3 % of the TSI. The spectral distribution of the extra-terrestrial radiation is such that about half of it lies in the visible part of the electromagnetic spectrum. It produces daylight and is well perceived by the human vision system. Other parts of it are in the near-infrared and ultraviolet ranges. A series of equations is offered to compute the extra-terrestrial total radiation for any instant and for any inclined surface. During its path downwards to the earth surface, the constituents of the atmosphere deplete the incident solar radiation. On average, less than half of extra-terrestrial radiation reaches ground level. A good knowledge of the optical properties of the atmosphere is necessary to understand and model the depletion of the radiation. The description and modelling of the optical processes affecting the solar radiation within the atmosphere is called radiative transfer. The phenomena of scattering and absorption are presented and the effects of molecules, aerosols, gases and clouds on radiation are discussed. Several examples are given that illustrate atmospheric effects as a function of the solar zenithal angle and atmospheric optical properties. Even when the sky is very clear with no clouds, approximately 20 % to 30 % of extra-terrestrial radiation is lost during the downwelling path by scattering and absorption phenomena by aerosols and molecules. The role of the clouds is of paramount importance: optically thin clouds allow a small proportion of radiation to reach the ground while optically thick clouds create obscurity by stopping the radiation downwards. The magnitude of the depletion of the radiation varies with wavelength and the spectral distribution of the solar radiation is modified as the radiation makes its path downwards. The spectral distribution is discussed for several different conditions. The direct, diffuse and reflected components of the solar radiation at earth surface are defined. How to compute them on an inclined surface is briefly discussed and equations are provided. The direct radiation is the radiation coming from the direction of the sun. Only direct radiation is present at the top of the atmosphere. On the contrary, the radiation at surface comprises a direct and a diffuse components, the sum being called the global radiation. If a tilted surface is under concern, then it may also receive a reflected component that is a part of the radiation reflected by the surrounding landscape. How to compute each component on an inclined surface is briefly discussed.