This review seeks to assist industrial hygienists in the prevention of Legionnaires' disease caused by Legionella bacteria. Breathing water droplets contaminated with Legionella bacteria, in which the organism has been permitted to amplify, causes this disease. Possible sources of transmission include nearly all manmade building water systems. Legionella organisms, found in most natural water sources but at very low concentrations, can thrive under conditions of warmth in these manmade systems. Primary prevention of Legionnaires' disease requires prevention of amplification of Legionella in water systems. This, in turn, requires familiarity with the system and all its components, and effective maintenance and water treatment. However, good maintenance and water treatment regimens alone cannot assure that amplification will not occur somewhere in the system. Systematic microbiological testing for Legionella and appropriate interpretation of the testing results can be powerful assets in prevention by enabling the detection and control of amplification. The occurrence of a confirmed or suspected case of Legionnaires' disease in a building occupant may indicate transmission within the facility; this poses an immediate crisis for the facility manager. An aggressive intervention is indicated to search for previously unknown additional cases of illness, to detect potential sources of transmission, and to decontaminate any suspected sources of transmission on an emergency basis. Once adequate remediation has been achieved and confirmed by microbiological testing, on-going control measures are essential with periodic microbiological investigation to assure continuing prevention of amplification.