Abstract The eight-million (16%) U.S. children who have a Behavior Disorder (BD), as well as the ten-fold increase in education, health care, and criminal justice costs associated with BDs, highlight the critical role of early intervention. Yet, there is variability in the potency and efficiency of Behavioral Parent Training (BPT) for early- onset (3 to 8 y.o.) BDs, suggesting the likelihood of a yet to be identified causal mechanism underlying treatment process and outcomes. One likely candidate, emotion regulation (ER), has been implicated in a broad range of adult and child outcomes, including early-onset BDs. For example, parental difficulties with ER, as well as difficulties helping children to navigate and regulate emotion, are linked to the etiology, maintenance, and severity of early-onset BDs. Preliminary data suggest that interpersonal ER processes may also underlie variability in the potency and efficiency of BPT for families of children with early-onset BDs. As such, this NIMH Exploratory and Developmental Research Grant (Parent R21) aims to further explore the interrelationship of interpersonal ER processes as a causal mechanism for BPT outcomes via the following aims: 1). Describe the dynamic nature of ER processes in families of children with early-onset BDs. 2). Examine the interrelationship of general ER, moment-by-moment ER in the parent-child context, and BPT potency and efficiency; and 3). Use exploratory findings to inform the development and testing of personalized BPT approaches based on the interrelationship of general ER, moment-by-moment ER in the parent-child context, and BPT potency and efficiency.