Prospective longitudinal SNF-funded project (Swiss National Science Foundation; grant no. 10001G_200803) on personality change surrounding romantic relationship transitions in young adults. Over 2.5 years, N = 1,781 participants (18–40; ~50% single/partnered at baseline) were followed in a measurement-burst design with seven waves: global assessments at wave start/end plus 3-day experience sampling. The major aims are (1) to capture short-term personality dynamics before, during, and after relationship transitions (e.g., beginning or ending a romantic relationship); (2) to integrate trait-, state-, and situational perspectives; (3) to test selection, anticipation, and socialization effects, that is, whether personality predicts who will begin or end a relationship, and whether personality changes before or after these relationship events; and (4) to examine how subjective event appraisals explain heterogeneity in personality change following relationship transitions.