Dutyholders responsible for workplaces or premises with relevant water systems must identify and assess reasonably foreseeable exposure to Legionella bacteria and decide what precautions are required. The HSE's Approved Code of Practice L8 and accompanying HSG274 technical guidance explain the legal and technical framework. Legionella pneumophila is found naturally in water sources and can multiply in artificial systems where conditions such as temperatures between 20°C and 45°C, stagnation, deposits and biofilm support growth. Inhalation of contaminated aerosols can cause Legionnaires' disease, a potentially fatal form of pneumonia. A suitable assessment examines relevant water assets, operating conditions, aerosol routes, exposed populations, existing controls and management arrangements. It should include or reference an accurate schematic, identify significant findings and explain any limitations. Where risk requires control, the dutyholder should prepare a site-specific written scheme defining responsibilities, precautions, monitoring, control limits and remedial action. L8 requires access to competent help, but the dutyholder remains accountable when work is contracted out. Assessments should be reviewed regularly and whenever there is reason to believe the original assessment may no longer be valid; UK guidance does not impose a universal fixed two-year expiry date.