PURPOSE: Little is known about potential influences on emerging partnered intimate behaviors in early adolescence. We investigate (1) the prevalence of partnered intimate activities and (2) associations with social relationships, parental monitoring and supervision, health behaviors, and psychosocial well-being. METHODS: We used population-based data from the UK's Millennium Cohort Study on 11,079 participants aged 14 years. Partnered intimate activities were grouped into three categories: "light" (handholding, kissing, and cuddling); "moderate" (touching and fondling under clothes); and "heavy" (oral sex and sexual intercourse). Multinomial logistic regression models were used. RESULTS: Thirty percent of study participants reported not engaging in partnered intimate activity. Fifty-eight percent reported "light," 7.5 percent "moderate," and 3.2 percent "heavy" activity. Associated with increased likelihood (adjusted relative risk ratios [RRRs]) of intimate activities were confiding worries in a friend (light RRR = 2.13, moderate RRR = 3.42, heavy RRR=5.32), low parental monitoring-staying out late or overnight (late: light RRR = 1.62, moderate RRR = 2.44, heavy RRR = 2.32; overnight: light RRR = 1.57, moderate RRR = 1.94, heavy RRR = 3.38), health-damaging behaviors (per unit increase: light RRR = 1.91, moderate RRR = 3.15, heavy RRR = 5.03), and depressive symptoms (per scale point increase light RRR = 1.03, moderate RRR = 1.09, heavy RRR = 1.11). Confiding in a parent was associated with lower likelihood of intimate activity (light RRR = .82, moderate RRR = .65, heavy RRR = .65). CONCLUSIONS: Partnered intimate activity of some form is commonplace among 14-year-olds in the United Kingdom. Given the short- and long-term implications of adolescent sexual development and well-being, improving our understanding of influences could help identify opportunities for interventions with benefits across the lifecourse.