Over 200,000 American children visit the emergency room annually after a playground injury. Among the most promising means to reduce playground injuries is through increased quantity and quality of playground supervision by adults. To implement empirically supported playground supervision interventions, it is essential to understand how supervision prevents injuries. Recent research suggests supervision likely prevents injuries through several different mechanisms; this paper discusses six: (a) Supervisors repetitively teach children playground rules; (b) supervisors recognize and stop children's dangerous behavior; (c) supervisors prevent children from behaving impulsively; (d) the presence of a supervisor causes children to behave differently; (e) supervisors change children's attribution of risk; and (f) supervisors have influence as modelers and persuaders of safe behavior. Theoretical implications for future work and applied implications for the development of playground safety interventions are discussed. © Springer Science+Business Media, Inc. 2006.