Photocatalysis is an astonishing process that can be used for several applications, such as photocatalytic degradation of various organic pollutants in wastewater, hydrogen production, air purification, and antibacterial activity. When compared with other methods, photocatalysis is fast developing and gaining extra attention owing to its various advantages, such as low cost and high efficiency. Photocatalysis is a unique process for rectifying energy and environmental issues. This chapter aims at semiconductor-based photocatalysis, that is, processes occurring on semiconductor-fluid interfaces while illuminated with the light of suitable energies. This chapter deals with basic principles, classification, mechanism, limitations, and operating parameters of photocatalytic processes. The theory and background necessary to understand heterogeneous photocatalysis are discussed, and emphasis is given on its electronic and optical properties. This is followed by the discussion of the principles of photocatalysis and thermodynamics as well as kinetic aspects, which determine the photocatalytic performance. The various types of available photocatalysts are presented along with issues related to quantum efficiency, response to light, and photochemical stability. A number of illustrative methods discussing metal oxide-and chalcogenide-mediated photocatalytic processes have been described mainly focusing on applications related to energy and the environment.