This article addresses Systems Theory as it applies to school-age children's bullying behavior. It focuses on the interrelationships, mutual influences, and dynamics of relationships within the family, and how these may affect children's behavior toward their peers. The theory helps to explain the ways family patterns are reflected in children's negative interactions with peers, particularly bullying behavior. As such, Systems Theory was used to guide development of the content and strategies that formed the family component of Friendly Schools Friendly Families, a whole-school bullying prevention intervention. The intervention was designed to systematically target parenting factors identified as protective of bullying behavior and other problem behaviors, including parent--child communication, parent modelling, parenting style, parent bullying attitudes and beliefs, normative standards about bullying, family management techniques, connectedness, and cohesion. This whole-school program thus actively engaged and enhanced the self-efficacy of both parents and teachers, and was found to be effective in reducing bullying behavior.