Developmental perspectives on self and identity view a sense of personal sameness and continuity as critical for positive adjustment. Thus, the degree to which individuals perceive change over time in self and direction constitutes an important individual difference. Here, we offer an empirically sound instrument for assessing the extent to which people feel temporally discrepant and off course—a sense we term derailment. First, we develop and empirically validate a self-report measure that is sensitive to our conceptualization of derailment (Studies 1–3). Employing the new measure with adult samples, Study 3 demonstrates its predictive ability above and beyond other widely used measures of subjective change and identity distress. Study 4 shows the negative effects of derailment persist independent of whether individuals perceive changing for the better or worse, or actually experience status-changing life events. Study 5 demonstrates the prospective utility of this measure by predicting depressive symptoms 18 months later. Finally, levels of derailment are shown to be reduced by a daily writing experiment that emphasizes goal continuity (Study 6). The discussion situates derailment at the intersection of developmental, clinical, and social psychological literatures as a unique and measurable source of psychological vulnerability, and strategies for attenuating its potentially deleterious impact are suggested.