We use a unique longitudinal data set to study the development of non-cognitive skills in adolescence. We measure – for the first time – the development over six years of the recently introduced non-cognitive skill “Grit.” We also measure the traditional Big Five personality traits. For Grit, we find significant within-person mean-level increases of about .5 standard deviation units for our sample of adolescent students. These increases are comparable with increases in the Big Five, where conscientiousness, agreeableness, and emotional stability also increase with similar magnitude. We show that these changes are heterogeneous and robust to reasonable measurement error.