Photochemistry (a term that broadly speaking includes photophysics) isa branch ofmodern science that deals with the interaction of light with matterand lies at the crossroads of chemistry, physics, and biology. However, beforebeing a branch of modern science, photochemistry was (and still is today),an extremely important natural phenomenon. When God said: “Let there belight”, photochemistry began to operate, helping God to create the world aswe now know it. It is likely that photochemistry was the spark for the origin oflife on Earth and played a fundamental role in the evolution of life. Throughthe photosynthetic process that takes place in green plants, photochemistryis responsible for the maintenance of all living organisms. In the geologicalpast photochemistry caused the accumulation of the deposits of coal, oil, andnatural gas that we now use as fuels. Photochemistry is involved in the controlof ozone in the stratosphere and in a great number of environmental processesthat occur in the atmosphere, in the sea, and on the soil. Photochemistry is theessence of the process of vision and causes a variety of behavioral responses inliving organisms.