Background: By focusing upon formal sex education programmes, the Mozambican government has significantly enhanced the general health of adolescents and young adults. However, when it comes to contraception, little is known about how adolescents and young adults actually behave. Methods: Based upon a qualitative study in two settings in Maputo province - Ndlavela and Boane - this paper explores the knowledge and practices of contraception among adolescents and young adults. A total of four focus group discussions, 16 in-depth interviews, four informal conversations, and observations were equally divided between both study sites. Results: Discrepancies between what adolescents and young adults know and what they do quickly became evident. Ambivalent and contradictory practices concerning contraceptive use was the result. As well, young people had numerous interpretations of risk-taking when not using contraceptives. These inconsistencies are influenced by social and medical barriers such as restricted dialogue on sexuality among adolescents and young adults and their parents and peers. Additionally, ideas about indigenous contraceptives, notions of masculinity and femininity, misconceptions and fear of the side effects of contraceptives, make people of all ages wary of modern birth control. Other barriers include imposed contraceptive choice - meaning no choice, overly technical medical language used at clinics and the absence of healthcare workers more attuned to the needs of adolescents and young adults. Conclusions: Adolescents and young adults have numerous - often erroneous - opinions about contraception, leading to inconsistent use as well as vague perceptions of risk-taking. Moreover, social norms and cultural gender roles often contradict and hinder risk-avoiding behaviour. Therefore, in order to improve young people's health, policymakers must address the reasons behind this ambivalence and inconsistency.