Research on the prevention of HIV/STIs generally has relied on self reports of sexual activity, which are vulnerable to bias. Self-reported data on sexual behaviours could have poor validity for several reasons, namely social desirability bias, recall bias, lack of awareness of exposure (eg, undetected condom breakage), and poor comprehension or misinterpretations of the survey questions. This presentation will briefly describe biomarkers of semen exposure, in particular, prostate-specific antigen detected in vaginal fluid, and will give examples of the ways in which biomarkers could be used to strengthen research on (1) effectiveness of barrier methods against HIV/STIs; (2) effectiveness of behavioural interventions to prevent HIV/STIs; (3) condom “migration” from HIV/STI interventions; (4) the validity of self-reported data; and (5) methods to improve the validity of self-reported data. Limitations of biomarkers include their narrow scope, cost, relatively quick clearance, and unknown biological significance of biomarker levels in relation to risk of HIV/STIs. Finally, examples of future areas of research will be provided.