Behavioral parent training (BPT) is a theory‐driven, evidence‐based, and widely used intervention strategy for preventing and decreasing children’s disruptive behavior problems, indirectly via improved parenting behavior. However, not all families benefit equally from BPT. To date, our knowledge of who benefits (and who does not) and our understanding of why some families benefit more than others is limited. An important challenge for research and practice is finding ways to tailor interventions to the needs of an individual family and increase their effectiveness. We put forward family systems theory as a tool to gain more insight into which families (do not) benefit from BPT and why. We synthesize the theoretical foundations and empirical support for the putative mechanisms through which the functioning of family systems may explain BPT effectiveness and propose ways in which family systems theory can help strengthen the design, implementation, and evaluation of BPT programs.