Research aimed at investigating sexual behaviour and assessing interventions to improve sexual health has increased in recent decades. The resulting data, despite regional differences in quantity and quality, provide a historically unique opportunity to describe patterns of sexual behaviour and their implications for attempts to protect sexual health at the beginning of the 21st century. In this paper we present original analyses of sexual behaviour data from 59 countries for which they were available. The data show substantial diversity in sexual behaviour by region and sex. No universal trend towards earlier sexual intercourse has occurred, but the shift towards later marriage in most countries has led to an increase in premarital sex, the prevalence of which is generally higher in developed countries than in developing countries, and is higher in men than in women. Monogamy is the dominant pattern everywhere, but having had two or more sexual partners in the past year is more common in men than in women, and reported rates are higher in industrialised than in non-industrialised countries. Condom use has increased in prevalence almost everywhere, but rates remain low in many developing countries. The huge regional variation indicates mainly social and economic determinants of sexual behaviour, which have implications for intervention. Although individual behaviour change is central to improving sexual health, efforts are also needed to address the broader determinants of sexual behaviour, particularly those that relate to the social context. The evidence from behavioural interventions is that no general approach to sexual-health promotion will work everywhere and no single-component intervention will work anywhere. Comprehensive behavioural interventions are needed that take account of the social context in mounting individual-level programmes, attempt to modify social norms to support uptake and maintenance of behaviour change, and tackle the structural factors that contribute to risky sexual behaviour.