For more than 20 years, the system of care philosophy has comprised a central strategy of service reform in children's mental health at the federal, state, tribal, and local levels. System of care terminology has become widely used in many different service sectors but often without a shared understanding of what a system of care is. This manuscript, the feature article for this special issue of "Evaluation and Program Planning," briefly traces the history of the system of care movement, discusses the more widely used definitions in the field of children's mental health, and offers an expanded version of the definition that takes into account the complex nature of systems of care. This manuscript was the product of a process used by the "Case Studies of System Implementation" research team to identify established system of care communities for inclusion in the research study. The purpose of this manuscript is to describe the process the research team engaged in while developing the revised definition and to open a public dialogue about how to characterize the essential properties of a system of care. (Contains 2 tables.)