Despite social, cultural and religious prohibitions on premarital sexual relationships in Iran, some evidence suggests that premarital heterosexual relationships and sex among young people may be increasing, but no study has been conducted which gives a comprehensive picture of perceived societal norms, attitudes, sexuality and the nature and process of heterosexual relationships among unmarried young females in Iran. This study aimed to investigate and compare the views and conduct of young female college students with perceived societal norms. The objectives included; exploring sexual norms and attitudes, assessing the social and individual factors responsible for variation in norms, attitude, and behaviour of female college students and finally exploring the process and circumstances of sexual behaviour. Complementary qualitative and survey methods were used. The qualitative research includes 4 Focus Group Discussions (FGDs) and 30 In-Depth Interviews (IDIs). FGDs explored societal norms on pre-marital sexuality, while IDIs explored more personal attitudes and behaviours in greater depth. The survey was conducted in 2005-6 on a sample of 1743 female undergraduate students in four multidisciplinary universities in Tehran. A self-administered questionnaire was applied. This study revealed an ambiguity with regard to social acceptability of premarital heterosexual relationships, while sexual contact before marriage was perceived as socially proscribed. Personal attitudes were considerably more tolerant of such premarital relationships and sex than societal norms. These findings suggest that traditional norms on heterosexual relationships are eroding among young people. Within this complex attitudinal context, nearly half of unmarried women reported premarital friendships with men and more than one-fifth reported any type of sex (23%). Low self-efficacy, peers' liberal norms on virginity, older age, parents' liberal attitude and poor family relationships were predictors of ever having sexual contact. Qualitative findings showed that gender double-standards dictate restrictions for women, but permit sexual licence for men and owing to men's preference for a virgin bride, most relationships which involve sex are unlikely to result in marriage. Therefore although young people have adopted some liberal values in their premarital relationships their marriage still follows traditional and cultural values. These results have implications for programmes designed to improve young people's reproductive health in Iran.