Objective: We evaluated the effectiveness of 4 parent-training programs for children with externalizing problems. We tested the effectiveness of 3 behavioral programs (Comet, Cope, and Incredible Years) and 1 nonbehavioral program (Connect) in reducing child behavior problems and attention-deficit/hyperactivity disorder (ADHD) symptoms, in improving positive parenting and parenting competence, and in decreasing negative parenting and parents’ stress and depressive symptoms. Method: This national study was designed as a randomized-controlled effectiveness trial (RCT). The treatments were carried out in 30 clinical and community-based practices. Parents of 908 children (ages 3–12 years) were randomly assigned to 1 of 2 parent training programs available at each practice, or to a wait-list condition, where parents had sought help from regular services. Before and after treatment, parents rated child behavior problems and parenting strategies. Results: At posttreatment, children whose parents had received interventions showed a strong decrease in child conduct problems and a moderate to strong decrease in ADHD symptoms. About half of parents whose children scored over the 95th percentile on the behavior measures (Eyberg Child Behavior Inventory, Swanson, Nolan, and Pelham Rating Scale), a clinically meaningful cutoff, reported that their children were no longer above the cutoff after the intervention. Parents showed considerably less negative behaviors toward their children at posttest compared with pretest; they increased in parental competence, and decreased in both stress and depressive symptoms. Overall, the behavioral programs were more effective than the nonbehavioral program. Conclusions: The results support the general efficacy of parent training in a short-term perspective.