Background: To counter the disproportionate impact of sexually transmissible infections (STIs) among young people and encourage higher levels of STI testing, it is necessary to identify the factors that influence STI testing. Methods: A mixed-methods study incorporating a cross-sectional quantitative survey and qualitative analysis of individual interviews was conducted in England. Some 275 university students aged 17-25 years completed an online questionnaire. Interviews were conducted with a purposively selected sample of eight men and women. Results: Multivariate analysis of quantitative data revealed that injunctive norms (i.e. a desire to comply with others' wishes for testing), descriptive norms (i.e. perceptions of others' behaviour) and shame related to STIs predicted past testing behaviour. Intention to undergo testing was predicted by greater perceived susceptibility, past testing, stronger injunctive norms and greater willingness to disclose sexual histories. Qualitative analysis of interview data confirmed the importance of perceived susceptibility, normative beliefs, stigma and shame, and perceived ease of testing. Conclusions: To increase STI testing among young people, there is a need to promote pro-testing norms, address low perceived susceptibility and make testing easier.