OBJECTIVE To examine the effects of child temperament and stressful family functioning on child behavior problems among preschool children. METHODS One hundred forty-five preschool children, aged 2 to 5 years, were evaluated by teachers, mothers, and independent observers. Teachers reported on child temperament; from these ratings, two dimensions of temperament were derived: difficult/easy and approachability. Mothers reported on two dimensions of family functioning: conflict and expressiveness. Both teachers and independent observers rated child behavior problems. RESULTS Children with more difficult temperaments who were in high-conflict families had the most internalizing and externalizing behavior problems, while children with easy temperaments had fewer such problems, regardless of levels of family conflict. Difficult children whose families were highly expressive engaged in the most observed aggression. CONCLUSIONS Results suggest that temperament is involved in both protective and vulnerability processes. A difficult temperament operates as a vulnerability factor for internalizing and externalizing behavior problems and observed aggression, while an easy temperament functions as a protective mechanism for these outcomes.