The current study examined trajectories of relationship confidence, defined as the belief that oneself and the partner together have the skills needed to navigate conflict and maintain a partnership into the future. This study uses data from a sample of 1,294 partnered but unmarried young adults to examine trajectories of relationship confidence across 11 waves of assessment over more than 4 years and the between- and within-person predictors of those trajectories. On average, relationship confidence was high at the outset of the study and remained stable over time. Underlying the overall stability, however, men's and women's trajectories flowed in opposite directions. Women started with more confidence than men, and their confidence decreased over time while men's increased, although the analysis of individuals rather than couples must be considered when interpreting this finding. Individuals in longer partnerships, who were cohabiting at Wave 1, with lower levels of avoidant attachment, more frequent positive interactions and higher satisfaction at Wave 1, and those who married during the study, had higher initial relationship confidence. Having children with a prior partner predicted lower initial confidence and faster decreases over time only for women. At the within-person level, relationship confidence was higher than usual at waves when more positive interactions, less negative interactions, and higher relationship satisfaction were reported, particularly for women. Results highlight the substantial variability in trajectories of relationship confidence among individuals in unmarried relationships as well as an array of factors that influence its development. (PsycINFO Database Record (c) 2020 APA, all rights reserved).