Background: The UNGASS target to reduce HIV prevalence by 25% among 15-25 year olds living in the most affected countries by 2005 has not been met. In the absence of a vaccine or cure, behavioural interventions are the main strategy for HIV control. The ability of specific behaviour change interventions to reduce HIV/STI incidence and unplanned pregnancies in young people remains unproven. Methods: Since January 1999, an adolescent sexual and reproductive health (SRH) intervention has been implemented in 10 randomly selected intervention communities in rural Tanzania, within a community randomised trial. The intervention consisted of teacher-led, peer-assisted in school education, youth-friendly health services, community activities and youth condom promotion and distribution. Process evaluation in 1999-2002 showed high intervention quality and coverage. From June 2007 to July 2008, in the specific research reported in this thesis, the long-term impact of the intervention was evaluated among 13,814 young people aged 17-27 years who had attended trial schools between 1999 and 2002. Findings: Prevalences of HIV and HSV2 were 1.8% and 25.9% in males and 4.0% and 41.4% in females, respectively. The intervention did not significantly reduce risk of HIV (males adjusted prevalence ratio(aPR)=0.91; 95%CI:0.50-1.65; females aPR=1.07; 95%CI:0.68-1.67) or HSV2 (males aPR=0.94;95%CI:0.77-1.15; females aPR=0.96; 95%CI:0.87-1.06). The intervention was associated with a reduction in number of lifetime sexual partners reported by males (aPR=0.87; 95%CI:0.78-0.97) and an increase in reported condom use at last sex with a non-regular partner among females (aPR=1.34; 95%CI:1.07-1.69). There was a clear and consistent beneficial impact on knowledge, but no significant impact on reported attitudes to sexual risk, reported pregnancies or other reported sexual behaviours. Interpretation: SRH knowledge can be improved and retained long-term, but this intervention had little effect on reported behaviour or HIV/STI prevalence. Youth interventions integrated within intensive, community-wide risk reduction programmes may be more successful and should be evaluated.