Membrane fouling in reverse osmosis (RO) systems occurs when contaminants accumulate on the membrane surface, leading to reduced performance and efficiency. This fouling can result from various substances, including organic matter, inorganic salts, bacteria, and scaling agents. Factors such as feed water quality, pressure, temperature, and flow rate significantly influence fouling rates. As fouling develops, it causes increased resistance to water flow, necessitating higher operational pressures and potentially leading to membrane damage. Regular monitoring and cleaning protocols are essential to mitigate fouling, extend membrane lifespan, and maintain optimal system performance in water purification applications.