Clinically meaningful emotional and behavioral problems are thought to be present beginning in infancy, and may be reliably assessed in children as young as 12 months old. However, few studies have investigated early correlates of emotional and behavioral problems assessed in infancy. The current study investigates the direct and interactive contributions of early infant and caregiver characteristics thought to play an important role in the ontogeny of behavior problems. Specifically, the study examines: (1) the links between temperamental reactivity across the first year of life and behavior problems at 18 months, (2) whether children high in temperamental reactivity are differentially susceptible to variations in maternal sensitivity, (3) the extent to which child temperamental risk or susceptibility may further be explained by mothers’ experiences of stressful life events (SLEs) during and before pregnancy. Data were collected from 322 Mexican American families during prenatal, 6-, 12-, 18-, and 24-week home interviews, as well as during 12- and 18-month lab interviews. Mother reports of SLEs were obtained between 23-40 weeks gestation; temperamental negativity and surgency at 6 weeks and 12 months; and internalizing and externalizing behaviors at 18 months. Maternal sensitivity during structured mother-infant interaction tasks at the 6-, 12-, 18-, and 24-week visits was assessed by objective observer ratings. Study findings indicated that maternal SLEs before birth were associated with more infant negativity across the first year of life, and that negativity in turn was associated with more internalizing problems at 18 months. Ecological stressors thought to be associated with sociodemographic risk factors such as low-income and ethnic minority status may begin to exert cascades of influence on children’s developmental outcomes even before birth.