This article examines the patterns, and consequences, of infant temperamental reactivity to novel sensory input in a large ( = 357; 271 in current analysis) and diverse longitudinal sample through two approaches. First, we examined profiles of reactivity in 4-month-old infants using the traditional theory-driven analytic approach laid out by Jerome Kagan and colleagues, and derived groups characterized by extreme patterns of negative reactivity and positive reactivity. We then used a theory-neutral, data-driven approach to create latent profiles of reactivity from the same infants. Despite differences in sample characteristics and recruitment strategy, we noted similar reactivity groups relative to prior cohorts. The current data-driven approach found four profiles: high positive, high negative, high motor, and low reactive. Follow-up analyses found differential predictions of internalizing, externalizing, dysregulation, and competence trajectories across 12, 18, and 24 months of life based on 4-month reactivity profiles. Findings are discussed in light of the initial formulation of early reactivity by Kagan and the four decades of research that has followed to refine, bolster, and expand on this approach to child-centered individual differences. (PsycInfo Database Record (c) 2024 APA, all rights reserved).