Photochemistry and photophysics, i.e. the study of the interaction between light and matter, is an everyday experience in our life both as a natural phenomenon and as a human developed technology.Within the scientific community, photochemistry is applied to a wide range of disciplines, from physical chemistry, to supramolecular chemistry, chemical biology, materials science and nanoscience. It is thus very important to understand the fundamental concepts at the basis of the interaction of light with molecules and to know what information can be gained by photophysical and photochemical techniques, as well as practical aspects for the application of these techniques