Casual sex, sex occurring outside committed romantic relationships, has become normative among U.S. college students, and is often feared to lead to negative wellbeing outcomes. Yet, longitudinal empirical evidence is scarce, and the available evidence is mixed, pointing toward potential moderators. Furthermore, definitions of casual sex vary across studies, sex differences are predicted but rarely examined, and wellbeing indicators other than self-esteem and depression are rarely studied. This dissertation reports on three studies, based on one longitudinal and weekly diary dataset, that address several of these gaps. A university-wide sample of 855 freshmen and juniors (from a pool of 6,500 students contacted through the university registrar) completed online surveys at the beginning of the semester (T1); 666 of those completed T2 (end of first semester), and 528 completed T3 (end of second semester). A subsample of 230 single T1 students also completed a 12-week long, weekly-diary study during the first semester. All models tested for sex differences and controlled for demographics. Anxiety, life satisfaction, and psychosomatic symptoms were examined in addition to self-esteem and depression. Study 1 explored effects of casual sex on wellbeing across different types of casual relationships (onetime, longer casual, or any) and intimacy level (kissing, genital touching, oral sex, and vaginal/anal intercourse). Links ranged from positive to negative to nonsignificant across wellbeing indicators, casual sex definitions, and gender, suggesting a complex relationship. Using self-determination theory (SDT), Study 2 showed that genital casual sex between T1 and T3 for nonautonomous reasons (i.e., due to self-imposed pressures, external contingencies, or lack of intentionality) was linked to lower wellbeing both within those who had casual sex, and compared to those without casual sex. Study 3 found that sociosexuality, a personality orientation toward casual sex, moderated the effect of casual sex on wellbeing over one academic year (Study 3a) and on a weekly basis (Study 3b). Sociosexually restricted students reported lower wellbeing following casual sex than after no casual sex; unrestricted students reported similar or higher wellbeing following casual sex. The research on casual sex effects on wellbeing needs to move beyond main effects, and into exploring moderators and mediators.