The matter-energy reaction is the basis of a variety of fundamental scientific phenomena we have witnessed in nature. Photoreaction is related to the interaction of photons and the molecules of a substance. When the necessary photon in the ultra-violet or visible range of the electromagnetic spectrum is absorbed by the materials in concern, it may convert different poisonous elements into harmless substances, such as water and carbon dioxide. The study of photon physics and chemistry is fundamental for our understanding of the world we live in. The basic physiological processes through which the living species maintain their life cycles are also somehow related to the different photochemical reactions. Photo-catalysis happens to be a low-cost, versatile, and environment-friendly method that deals with a variety of harmful pollutants. Pollutants can be inorganic, organic, or even biological, and they can be found in both air and water. Photo-catalysis is a process in which the catalyst, light source, and contaminants must be in close proximity or contact. There have been numerous studies on the oxidation-induced removal of different organic pollutants as well as microorganisms, especially those found in water. In this chapter, we have covered the basics of photo-catalysis, the characteristics of various catalysts, their types, and photochemical laws, as well as the conditions and limitations of quantum yield.