Cooling towers reject heat efficiently, but warm recirculating water, extensive wetted surfaces and the release of fine droplets mean that poor control can create a serious Legionella risk. A suitable management programme must control microorganisms, scale, corrosion and fouling while maintaining safe operation and heat-transfer performance. The main technical reference in Great Britain is the Health and Safety Executive's HSG274 Part 1, supporting the Approved Code of Practice L8. The risk assessment and written scheme should cover the complete system, including make-up water, pre-treatment, circulation, tower cells, pack, basin, dosing equipment, bleed control, sampling points, standby sections and shutdown arrangements. Chemical treatment should be designed from current make-up water analysis, system materials, operating conditions and environmental constraints. Monitoring should verify chemical delivery and outcomes through representative chemistry tests, equipment checks, visual inspection and microbiological results. HSG274 Part 1 states that biocide effectiveness should be monitored weekly and that specific Legionella sampling should be carried out at least quarterly, with more frequent sampling where circumstances require it. Closed heating and chilled-water circuits need a separate inhibitor programme based on their own design, materials and operating conditions. Inhibitor concentration, pH, conductivity, corrosion indicators, glycol and microbiological activity should be interpreted as trends, with causes investigated before simply adding more treatment product.