The sun radiates a tremendous amount of energy, called solar energy or solar radiation, which is the main natural source of energy on the Earth, by far. Because solar radiation is the almost unique supplier of energy to the Earth, it has a primary influence on life and activities on the Earth. The climate is a first example, but there are many others, such as plant growth or human health, or even the design of buildings, the production of energy, notably electrical and thermal, or even aging materials. This book aims to provide simple answers to anyone who has questions about solar radiation. Its ambition is to help by presenting the fundamental elements of the solar radiation received on the ground. The book includes many examples and numerous illustrations, as well as some simple but fairly precise equations to calculate the various elements covered and to reproduce the figures and graphs. The first of the three parts of this book is devoted to the relative geometry between the direction of the sun and an observer on the ground as well as to the solar radiation emitted by the sun and received at the top of the atmosphere. The orbit of the Earth around the sun and the solar declination are described. The concept of time is introduced which is closely linked to the solar cycle and the rotation of the Earth on itself. Equations are given to calculate the solar radiation received on a horizontal or inclined surface located at the top of the atmosphere. The spectral distribution of the extraterrestrial solar radiation is described. The second part of this book addresses how the solar radiation incident at the top of the atmosphere is attenuated and modified in its downward path to the ground. The reflection of the radiation by the ground is presented. The solar radiation received on the ground by a horizontal or inclined collector plane, such as a natural slope or a rooftop, is discussed, as well as its spectral distribution. The variability of the radiation is addressed in relation to the properties of solar radiation estimated from the measurements. The third part deals with direct or indirect measurements of the solar radiation received on the ground over a given integration time (minute, hour, day, or month), whether for total radiation or radiation in a spectral range such as ultraviolet (UV), or daylight, or photosynthetically active radiation (PAR). It also explains how to check the plausibility of the measurements. Fundamentals of Solar Radiation will be a valuable resource to all professionals, engineers, researchers, students, and other practitioners that seek an understanding of solar radiation.