Surveys of sexual behaviour and prevention needs among men who have sex with men (MSM) have been primarily conducted on national levels. Due to different sampling methods (particularly Internet-based vs. venue-based) across countries, and because of similar, but not identical questions, a harmonisation of indicators is needed to allow, among other things, reporting of comparable data to the European Centre for Disease Prevention and Control (ECDC). In the last decade, websites for MSM social networking and other purposes have been shown to be (cost-)effective and valid means for recruiting respondents, and are thus increasingly used for surveying MSM in particular.