To create a fairer, more prosperous and gender-equal world there is the need to invest more in adolescent girls. This brief summarises two pieces of research showing how strategic investments in a package of 'accelerator' interventions for adolescent girls can support significant returns across educational attainment and learning, health promotion and violence prevention, and girls’ participation and decision-making. First, drawing on findings from 28 cost-benefit analyses, we provide a comprehensive synthesis and analysis of the expected rates of return for adolescent sexual and reproductive health and rights (SRHR) services, cash transfers, and parenting programmes globally (see Figure 1a) – based on significant existing evidence of the importance of these interventions for health, poverty, education, violence and empowerment. 1–3 Our analysis indicates that investing USD 1 billion in these interventions is likely to achieve more than a four-fold return on investment. Second, we provide a national-level analysis of the far-reaching and gender- transformative impacts from scaling-up the same three services in Kenya (see Figure 1b). Our findings indicate that over ten years, an investment of USD 234 million in adolescent girls, would avert 120,000 teenage pregnancies, 2,660 new HIV infections, 5,920 instances of child marriage, 8,510 cases of sexual violence, and 54,400 cases of emotional or physical violence. Additionally, this investment would support an extra 226,000 years of schooling and lead to a USD 886 million return from increased labour market productivity.