Parenting stress and children’s behavior problems have frequently been linked, with bidirectional relations spanning from early childhood through adolescence. However, this association has not been well studied in infancy or toddlerhood, and prospective mediators have not been thoroughly explored. This prospective, longitudinal study utilized two transactional models to examine bidirectional relations between parenting stress and children’s behavior problems and explore perceived family conflict and parental supportiveness as potential mediators. Data were taken from the Early Head Start Family and Child Experiences Study, where 835 parent–child dyads were assessed at 1, 2, and 3 years. Parenting stress and behavior problems were measured at all 3 time points, while family conflict and observed parental supportiveness were measured at ages 2 and 3. Results indicated that parenting stress and children’s behavior problems were relatively stable over time and had bidirectional or cross-lagged associations. Family conflict mediated the relation between children’s behavior problems at age 1 and parenting stress at age 3, while parental supportiveness mediated the relation between parenting stress at age 1 and behavior problems at age 3, suggesting both “child” and “parent” effects that function through two different mechanisms. These findings suggest that early prevention programs should focus on both children’s behavior and parenting stress in the first year and work to reduce family conflict and increase parental supportiveness in order to disrupt this negative cycle. (author abstract)