Background: Since nearly half of new HIV infections worldwide occur among young people aged 15-24 years, changing sexual behaviour in this group will be crucial in tackling the pandemic. Qualitative research is starting to reveal how social and cultural forces shape young people's sexual behaviour and can help explain why information campaigns and condom distribution programmes alone are often not enough to change it. We undertook a systematic review to identify key themes emerging from such research, to help inform policymakers developing sexual health programmes, and guide future research.

Methods: We reviewed 268 qualitative studies of young people's sexual behaviour published between 1990 and 2004. We developed a method of comparative thematic analysis in which we coded each document according to themes they contained. We then identified relations between codes, grouping them accordingly into broader overall themes. Documents were classified as either primary or secondary depending on their quality and whether they contained empirical data. From the 5452 reports identified, we selected 246 journal articles and 22 books for analysis.

Findings: Seven key themes emerged: young people assess potential sexual partners as "clean" or "unclean"; sexual partners have an important influence on behaviour in general; condoms are stigmatising and associated with lack of trust; gender stereotypes are crucial in determining social expectations and, in turn, behaviour; there are penalties and rewards for sex from society; reputations and social displays of sexual activity or inactivity are important; and social expectations hamper communication about sex. The themes do not seem to be exclusive to any particular country or cultural background, and all themes were present, in varying degrees, in all countries assessed.

Interpretation: This study summarises key qualitative findings that help in understanding young people's sexual behaviour and why they might have unsafe sex; policymakers must take these into account when designing HIV programmes. Considerable overlap exists between current studies, which indicates the need to broaden the scope of future work.