This study draws together survey and qualitative data on sexual practices among more than 1750 young Northern Thai people aged 17–20 years. The survey data indicate that sexually active young people frequently engage in, or are subjected to, risk-taking behaviours that may expose them to sexually transmitted infections and unwanted pregnancies. These include having multiple sexual partners and quite frequent partner turnover. High percentages also engage in unprotected sexual intercourse with various types of sexual partner (steady, casual and paid), and young women especially had often experienced sexual coercion. Qualitative data revealed a mixture of perceptions and practices affecting sexual intercourse among the young, such as having unplanned sex, engaging in sexual relations to display love or cement committed relationships, and having serial relationships, both monogamous and non-monogamous. We conclude that condom use should be a central focus of activities aimed at preventing adverse sexual health outcomes, but that new intervention approaches to encourage use of other contraceptives are also needed. Changes in sexual norms among young people also need to be acknowledged and accepted by older Thai generations in order for programmes and interventions to combat negative sexual and reproductive health consequences to be more effective.