This paper updates evidence reviewed in the first edition of Getting Australia Active on effective physical activity (PA) intervention strategies among children, adolescents and young adults. Intervention studies published between 1999 and September 2003 were identified using electronic databases and hand searching. A total of 28 discrete studies were identified (31 papers). Six of nine studies reported significant effects on child or youth PA in school settings. Those that incorporated whole-of-school approaches including curriculum, policy and environmental strategies appeared to be more effective than those that incorporated curriculum-only approaches. Five of 10 studies with children and two of five studies with adolescents reported increased PA or decreased sedentary behaviour in other settings. Interventions that included contact with families generally appeared to be most effective. One study with adolescents provided some evidence of the potential effectiveness of interventions based in primary care. Two of four papers reported modest short-term results among young adults, including increased PA stage of change or a higher likelihood of being adequately physically active, but none showed any evidence of sustained increases in PA. There is an urgent need for additional studies examining interventions aimed at young adults. Across the three life stages, future studies should include objective PA measures, longer-term follow-up, larger sample sizes, a specific focus on PA (rather than weight) and culturally-specific strategies that build evidence in Australian populations. Future studies should target high risk groups and a broad range of settings and strategies focusing on reducing sedentary behaviours as well as increasing PA.