Background Unintended pregnancy among adolescents represents an important public health challenge in high-income countries, as well as middle- and low-income countries. Numerous prevention strategies such as health education, skills-building and improving accessibility to contraceptives have been employed by countries across the world, in an effort to address this problem. However, there is uncertainty regarding the effects of these interventions, hence the need to review the evidence-base. Objectives To assess the effects of primary prevention interventions (school-based, community/home-based, clinic-based, and faith-based) on unintended pregnancies among adolescents. Search methods We searched all relevant studies regardless of language or publication status up to November 2015. We searched the Cochrane Fertility Regulation Group Specialised trial register, The Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2015 Issue 11), MEDLINE, EMBASE, LILACS, Social Science Citation Index and Science Citation Index, Dissertations Abstracts Online, The Gray Literature Network, HealthStar, PsycINFO, CINAHL and POPLINE and the reference lists of articles. Selection criteria We included both individual and cluster randomised controlled trials (RCTs) evaluating any interventions that aimed to increase knowledge and attitudes relating to risk of unintended pregnancies, promote delay in the initiation of sexual intercourse and encourage consistent use of birth control methods to reduce unintended pregnancies in adolescents aged 10 years to 19 years. Data collection and analysis Two authors independently assessed trial eligibility and risk of bias, and extracted data. Where appropriate, binary outcomes were pooled using a random-effects model with a 95% confidence interval (Cl). Where appropriate, we combined data in meta-analyses and assessed the quality of the evidence using the GRADE approach. Main results We included 53 RCTs that enrolled 105,368 adolescents. Participants were ethnically diverse. Eighteen studies randomised individuals, 32 randomised clusters (schools (20), classrooms (6), and communities/neighbourhoods (6). Three studies were mixed (individually and cluster randomised). The length of follow up varied from three months to seven years with more than 12 months being the most common duration. Four trials were conducted in low-and middle-income countries, and all others were conducted in high-income countries. Multiple interventions Results showed that multiple interventions (combination of educational and contraceptive-promoting interventions) lowered the risk of unintended pregnancy among adolescents significantly (RR 0.66, 95% CI 0.50 to 0.87; 4 individual RCTs, 1905 participants, moderate quality evidence. However, this reduction was not statistically significant from cluster RCTs. Evidence on the possible effects of interventions on secondary outcomes (initiation of sexual intercourse, use of birth control methods, abortion, childbirth, sexually transmitted diseases) was not conclusive. Methodological strengths included a relatively large sample size and statistical control for baseline differences, while limitations included lack of biological outcomes, possible self-report bias, analysis neglecting clustered randomisation and the use of different statistical tests in reporting outcomes. Educational interventions Educational interventions were unlikely to significantly delay the initiation of sexual intercourse among adolescents compared to controls (RR 0.95, 95% CI 0.71 to 1.27; 2 studies, 672 participants, low quality evidence). Educational interventions significantly increased reported condom use at last sex in adolescents compared to controls who did not receive the intervention (RR 1.18, 95% CI 1.06 to 1.32; 2 studies, 1431 participants, moderate quality evidence). However, it is not clear if the educational interventions had any effect on unintended pregnancy as this was not reported by any of the included studies. Contraceptive-promoting interventions For adolescents who received contraceptive-promoting interventions, there was little or no difference in the risk of unintended first pregnancy compared to controls (RR 1.01, 95% CI 0.81 to 1.26; 2 studies, 3,440 participants, moderate quality evidence). The use of hormonal contraceptives was significantly higher in adolescents in the intervention group compared to those in the control group (RR 2.22, 95% CI 1.07 to 4.62; 2 studies, 3,091 participants, high quality evidence)