OBJECTIVE: To assess progress in meeting young people's sex education needs in Britain by examining the current situation and changes over the past 20 years in sources of information about sexual matters and unmet information needs. DESIGN: Cross-sectional probability sample surveys. SETTING: British general population. PARTICIPANTS: 3869 men and women aged 16-24 years, interviewed 2010-2012 for the third National Survey of Sexual Attitudes & Lifestyles (Natsal-3), compared with 16-24 year-olds in Natsal-1 (1990-1991; 792 men and women) and Natsal-2 (1999-2001; 2673 men and women). MAIN OUTCOME MEASURES: Reported source of information about sexual matters, unmet information needs and preferred source of additional information. RESULTS: Between 1990 and 2012, the proportion citing school lessons as their main source of information about sexual matters increased from 28.2% (95% CI 24.6 to 32.1) to 40.3% (95% CI 38.6 to 42.1). In 2010-2012, parents were reported as a main source by only 7.1% (95% CI 5.8 to 8.7) of men and 14.1% (95% CI 12.6 to 15.7) of women and, for women, were less commonly reported than in 1999-2001 (21.7%; 95% CI 19.6 to 24.0). Most young people reported not knowing enough when they first felt ready for sexual experience (68.1% men, 70.6% women), and this did not change substantially over time. They wanted more information about psychosexual matters (41.6% men, 46.8% women), as well as sexually transmitted infections (27.8% men, 29.8% women) and, for women, contraception (27.5%). Young people primarily wanted this information from school, parents or health professionals. CONCLUSIONS: Over the past 20 years, young people have increasingly identified school lessons as their main source of information about sex, although they continue to report needing more information on a broad range of topics. The findings support the expressed need for improved sex and relationships education in schools alongside greater involvement of parents and health professionals.