Chlorine is the most widely used biocide and disinfectant in the world, valued for its strong oxidizing properties and effectiveness against a broad range of microorganisms including bacteria, viruses, fungi, and algae. Typically applied in the form of chlorine gas, sodium hypochlorite solution, or calcium hypochlorite solid, it is a cornerstone of municipal drinking water and wastewater treatment, ensuring safe water supplies by destroying harmful pathogens. Chlorine dissolves in water to form hypochlorous acid (HOCl), the active disinfecting agent, which penetrates microbial cell walls and disrupts essential biological processes, leading to cell death. Its effectiveness is highest around neutral pH levels, making it versatile for a wide range of applications from swimming pool sanitation to industrial cooling water systems and food processing facilities. Despite its benefits, chlorine use requires careful management due to its toxicity and the potential formation of disinfection by-products like trihalomethanes, which pose health risks at high levels. Storage and handling demand corrosion-resistant equipment and protective measures, as chlorine gas is hazardous to human health. Nevertheless, given its cost-effectiveness, broad-spectrum activity, and regulatory acceptance, chlorine remains the dominant biocide globally, playing a vital role in public health protection and industrial water management.