Transactional sex, the casual exchange of sexual favors for money or gifts, has been associated with negative outcomes and health risks, particularly among youth. This global review of the evidence explores trends of buying and selling of sex among the general population of male and female youth across 28 countries. It compares the differences and similarities in prevalence rates between genders (male versus female), sex trading activities (selling versus buying), and country income groups (high-income versus low and middle-income countries) and examines the relationships and situations surrounding transactional sex, and its correlates. The screening of reports resulted in the inclusion of 37 manuscripts (N=120,447 participants), involving peer review and grey literature describing longitudinal and cross-sectional research across 7 high-income and 21 low- and middle-income countries. The review of prevalence rates suggests relatively low rates of transactional sex in high-income countries (with selling and buying rates below 10% in all countries) and relatively high, although varying rates, in low- and middle-income countries (with selling and buying rates of 60% or higher in 7 countries). Gender disaggregated data suggests that boys are more likely than girls to sell sex in high-income countries while the opposite seems to be true in low- and middle-income countries. The findings suggest that initial contact between sellers and buyers is most often established through friends, acquaintances, and dating websites. The age of onset is around 15 years, many sellers and buyers already know each other before trading sex, and they are often of a similar age. Money is the most commonly used form of compensation. Correlates of selling sex include involvement in other risky sexual behaviors, substance use, infection with sexually transmitted diseases, mental health problems, family break-up, and a history of victimization. No or mixed relations have been found with socioeconomic and educational status. The correlates of buying sex include promiscuity, substance use, violence perpetration and, to some extent, higher socioeconomic status. Recommendations for future research are discussed.