The college transition is uniquely challenging for many first-year students. Few studies have investigated developmental change in students' adjustment across this brief, but significant transition, nor the daily interpersonal dynamics that are associated with adjustment across this same time. Guided by ecological and stage-environment fit frameworks, this study examined trajectories of first-year students' positive and negative affect across the transition to college. Further, we examined daily interactions with parents and friends as predictors of these trajectories. Participants were 146 first-year college students from a large southwestern university entering their first semester of college (Mage = 17.82, SD = 0.50). Electronic ecological momentary assessments (EMAs) were administered to students twice weekly (maximum 49 observations) from July to December to assess daily experiences during the transition to college and across the first semester. Multilevel growth analyses showed that students reported a meaningful decrease in positive affect across the first semester, but stable levels of negative affect. Involvement and conflict with parents and friends predicted variability in these average changes, as well as daily affective states. As expected, greater involvement with parents and friends was associated with greater positive and less negative affect, and reports of conflict with parents and friends predicted negative affect experiences. Together, these findings suggest the importance of support from parents and friends during the initial adaptation to college, as well as the potential undermining role of conflict with significant others. (PsycINFO Database Record (c) 2018 APA, all rights reserved).