A Legionella risk assessment is the foundation of a defensible water safety programme. It should identify how people could be exposed to Legionella bacteria, evaluate whether existing controls are effective and give the dutyholder a practical basis for preventing or controlling the risk. In Great Britain, the central references are the Health and Safety Executive's Approved Code of Practice L8 and the supporting technical guidance HSG274. Duties arise primarily under the Health and Safety at Work etc. Act 1974 and the Control of Substances Hazardous to Health Regulations 2002. A suitable and sufficient assessment should define its scope, identify the dutyholder and competent responsible person, describe the water systems, include an accurate schematic, examine conditions that support bacterial growth, identify routes of aerosol exposure, consider susceptible users and evaluate management arrangements. Where a reasonably foreseeable risk requires control, the dutyholder should prepare and implement a site-specific written scheme. The scheme should define correct operation, precautions, monitoring locations and frequencies, control limits, responsible people, record arrangements and the remedial action required when controls fail. Assessments should be reviewed regularly and whenever there is reason to believe they may no longer be valid; UK guidance does not impose a universal fixed expiry date. Monitoring and sampling should verify the chosen control strategy rather than replace competent assessment and management.