Background: This thesis involved the development and evaluation of a contraceptive behavioural intervention delivered by mobile phone for young people in Tajikistan, Palestine and Bolivia. Methods: The intervention was developed using behavioral science and evaluated by randomised controlled trial in each country. Outcome data were self-reported at four months. The primary outcome was acceptability of at least one method of effective contraception (N = 570 in Palestine and Tajikistan). In Bolivia, a co-primary outcome was use of effective contraception (N = 1310). Secondary and process outcome data were collected. I conducted a post-hoc change from baseline to follow-up analysis in Tajikistan and Bolivia. Interviews with trial participants were also conducted. Results: Intervention development: the results of the intervention development were similar across the countries. The interventions consist of short messages delivered over four months and include the same ten behaviour change methods. Tajikistan trial: 573 were enrolled and 82% (n = 472) completed follow-up. Intervention content was included on the app, causing contamination. Acceptability: 66% intervention vs 64% control; adjusted OR 1.21 95% CI .80-1.83, p = 0.36. Increase in acceptability from baseline to follow-up: 2% to 65%, p < 0.001. Palestine trial: 578 were enrolled and 80% (n = 464) completed follow-up. Acceptability: 31% intervention vs 17% control; adjusted OR 2.34, 95% CI 1.48-3.68, p < 0.001. Bolivia trial: 640 were enrolled and 67% (n = 429) completed follow-up. Use: 37% intervention vs 33% control; adjusted OR 1.19, 95% CI .80-1.77, p = 0.40. Acceptability: 71.92% intervention vs 62.56% control; adjusted OR 1.49, 95% CI .98-2.28, p = 0.06. Increase in acceptability from baseline to follow-up: 9% to 67%, p < 0.001. Interviews: interviewees highly valued the intervention. Conclusion: The interventions were well-specified, theory-based and tailored to each country. It is likely that the intervention delivered by short messages improves attitudes towards effective contraception.