Voluntary management systems (MS) might improve occupational health and safety (OHS). MSs, as with any OHS management, requires worker influence (and management commitment) to reduce risks at work. Influence through union backed safety representatives (or similar) achieves the best OHS results. However, a MS guarantees neither effective OHS management nor strong worker influence. Systematic violations of legal requirements have repeatedly caused accidents at workplaces with certified MSs. Why employers introduce MS's can affect their implementation and outcomes. Internal objectives, of productivity and/or work-related health, require upstream prevention and a genuine influence by workers and their safety representatives. However, managers with external objectives for a MS, such as brand images or low reported accident figures, may pursue such objectives through downstream control of safe behaviour - sometimes suppressing accident reporting - and with little attention to more serious long-term diseases. Worker consultation may then be a limited means of enhancing safety, or to one-way communication on safety rules. Such consultation may even be used for union busting. Many trade unions welcome MSs as possible improvement instruments, but they are also wary of what MSs employers are actually trying to implement. Unions try to support their safety representatives' influence in a MS by enhancing their rights and their competences. If the motives for MSs are mainly external and the opportunities for influence too limited, it may be better for unions not to cooperate with management in a MS that may lead more to manipulation than to safe and sound work.