Studies have shown that improved parenting mediates treatment outcomes for aggressive children, but we lack fine-grained descriptions of how parent-child interactions change with treatment. The current study addresses this gap by applying new dynamic systems methods to study parent-child emotional behavior patterns. These methods tap moment-to-moment changes in interaction processes within and across sessions and quantify previously unmeasured processes of change related to treatment success. Aggressive children and their parents were recruited from combined Parent Management Training and Cognitive-behavioral programs in "real world" clinical settings. Behavioral outcomes were assessed by reports from parents and clinicians. At pre- and post-treatment, home visits were videotaped while parents and children discussed consecutively: a positive topic, a mutually unresolved problem, and another positive topic. Results showed that significant improvements in children's externalizing behavior were associated with increases in parent-child emotional flexibility during the problem-solving discussion. Also, dyads who improved still expressed negative emotions, but they acquired the skills to repair conflicts, shifting out of their negative interactions to mutually positive patterns.