The present study investigated the developmental precursors of effortful control, a temperament trait that involves the propensity to regulate one's impulses and behaviors, to motivate the self toward a goal when there are conflicting desires, and to focus and shift attention easily. Data came from the California Families Project, a multimethod longitudinal study of 674 Mexican-origin youth (and their parents), who were assessed at ages 10, 12, 14, 16, and 19. Effortful control (measured via self- and parent-reports) was moderately stable over time (r = .47 from age 10 to 19), and its developmental trajectory followed a u-shaped pattern (decreasing from age 10 to 14, before increasing from age 14 to 19). Findings from latent growth curve models showed that youth who experience more hostility from their parents, associate more with deviant peers, attend more violent schools, live in more violent neighborhoods, and experience more ethnic discrimination tend to exhibit an exacerbated dip in effortful control. In contrast, youth with parents who closely monitor their behavior and whereabouts exhibited a shallower dip in effortful control. Analyses of the facets of effortful control revealed important disparities in their trajectories; specifically inhibitory control showed linear increases, attention control showed linear decreases, and activation control showed the same u-shaped trajectory as overall effortful control. Moreover, most of the precursors of effortful control replicated for inhibitory control and attention control, but not for activation control. We discuss the broader implications of the findings for adolescent personality development and self-regulation. (PsycInfo Database Record (c) 2020 APA, all rights reserved).