A stratified probability sample survey of the British general population, aged 16 to 44 years, was conducted from 1999 to 2001 (N = 11,161) using face-to-face interviewing and computer-assisted self-interviewing. We used these data to estimate the population prevalence of masturbation, and to identify sociodemographic, sexual behavioral, and attitudinal factors associated with reporting this behavior. Seventy-three percent of men and 36.8% of women reported masturbating in the 4 weeks prior to interview (95% confidence interval 71.5%-74.4% and 35.4%-38.2%, respectively). A number of sociodemographic and behavioral factors were associated with reporting masturbation. Among both men and women, reporting masturbation increased with higher levels of education and social class and was more common among those reporting sexual function problems. For women, masturbation was more likely among those who reported more frequent vaginal sex in the last four weeks, a greater repertoire of sexual activity (such as reporting oral and anal sex), and more sexual partners in the last year. In contrast, the prevalence of masturbation was lower among men reporting more frequent vaginal sex. Both men and women reporting same-sex partner(s) were significantly more likely to report masturbation. Masturbation is a common sexual practice with significant variations in reporting between men and women.