Young people's need for sex education is evidenced by their typically early initiation of sexual activity, the often involuntary context within which they have sexual intercourse, high-risk sexual behaviours and the inadequate levels of knowledge of means of protecting their sexual health. The earliness of initiation of sexual intercourse has implications for the age by which sexuality education should be provided. The extent and context of sexual behaviour is a firm indicator of the need for sex education as well as for counselling, information and services related to sexual and reproductive health. Apart from behaviours, information on the extent of knowledge and accuracy of knowledge about risks to sexual health and about means of preventing unhealthy or undesired outcomes are important indicators of young people's need for information to help them make choices and to engage in safe and healthy behaviours. Such measures of behaviour and knowledge can also be relevant and valid indicators of the effectiveness of sex education interventions. The context with which young people live and key characteristics such as school attendance and literacy are important considerations in providing information and in evaluating interventions. (Contains 6 tables.)