Water bodies receive large quantities of industrial, municipal, and agricultural wastewater loaded with diverse organic pollutants. Prominent examples include dyes from the textile industry, antibiotics and analgesics from pharmaceutical waste, pesticides, and phenolic compounds from petrochemical industries. These compounds are dangerous because of their toxicity, persistence, and tendency to bio accumulate; some, such as antibiotics, also contribute to the spread of microbial resistance. Conventional treatment plants do not fully remove these pollutants. Biological treatment is limited by the toxicity of the compounds and their slow degradation, while adsorption and precipitation merely transfer the pollutant from one phase to another without destroying it. This has driven interest in advanced oxidation processes, which generate highly oxidizing species such as hydroxyl radicals (•OH) and superoxide anions (•O₂⁻) capable of breaking down stable organic bonds. Heterogeneous photo catalysis can operate at ambient temperature and pressure, uses light as its energy source, and can achieve complete mineralization of pollutants. Progress in nanomaterial science now allows the design of composite photocatalysts that overcome the limitations of single-component catalysts. Wastewater contaminated with persistent organic compounds such as dyes, pharmaceuticals, pesticides, and phenols poses a growing environmental and public health challenge, since many of these compounds resist conventional biological and chemical treatment. Advanced oxidation processes, chief among them heterogeneous photocatalytic degradation, offer a promising route for mineralizing these pollutants into carbon dioxide, water, and inorganic ions. This article reviews the fundamental principles of photocatalytic degradation, the main classes of nanocomposite materials used (based on TiO₂, ZnO, g-C₃N₄, carbon materials, and heterojunctions), the factors that govern efficiency, the challenges that hinder the transition from laboratory to real-world application, and future research directions. 2. Fundamental Principles of Photo catalytic Degradation When a semiconductor absorbs photons with energy equal to or greater than its band gap, electrons are excited from the valence band to the conduction band, leaving positively charged holes in the valence band. These charges then migrate to the catalyst surface and react with adsorbed species: • Holes (h⁺) react with water or hydroxide ions to produce hydroxyl radicals, or oxidize the pollutant directly. • Electrons (e⁻) reduce dissolved oxygen to superoxide, which may later convert into other reactive species such as hydrogen peroxide. • These reactive species attack the organic molecules and break them down gradually through intermediate products, ultimately yielding CO₂ and H₂O. Three steps govern the overall efficiency: light absorption, charge separation and transport, and surface reactions. Electron–hole recombination before the charges reach the surface is the largest source of efficiency loss in single-component photo catalysts.