The system of care approach as a strategy for serving children and adolescents with serious emotional disturbance (SED) is by any measure a success with one exception: there is controversy regarding the scientific evidence documenting that the services provided through systems of care improve the symptomatic and functional outcomes of the youth and their families served when compared to the services provided through more traditional service systems. This paper traces this essential problem to challenges in the definitions of systems of care, particularly the level at which systems of care are conceived to exist and the impact of these challenges on the collection of relevant and meaningful data that could otherwise create a cumulative science regarding systemic interventions for youth with SED. In many regards, systems of care are often viewed in the context of programs of care that are predominantly evaluated within program evaluation rather than system evaluation perspectives. This article elucidates the problems created by the varying definitions of systems of care for the development of a cumulative practice and policy relevant research base pertaining to children and adolescents with serious emotional disturbance. Alternative strategies for future research are discussed in the context of alternative definitions of the system of care concept.