Measuring complex systems is a major challenge confronting children's mental health and other human service systems. Within the past 25 years, there has been increasing recognition that effective services for children and youth with serious emotional disturbances and their families requires, for example, a range of services; collaboration between a variety of service sectors, multiple organizations, parents and professionals; attention to careful planning; adaptation to change; performance measurement; and continuous quality improvement. While it has taken time for the field to distinguish these and other nuanced and multifaceted features of an effective system of care, the greater challenge may be to measure how well communities are doing in implementing such systems. The field is much more adept at measuring characteristics of individuals, or of discrete programs, than measuring complex systems with multiple components that connect and interact with each other in myriad and changing ways. The difficulty in measuring complex systems has held back progress in the field. The absence of an approach that can be used to assess how well individual communities and states are doing in implementing systems of care, in what areas they are doing well and not so well, and what the status of systems of care nationally has slowed progress in successfully developing such systems.The articles in this Special Section of the Journal of Behavioral Health Services & Research examine the national level of implementation of the core concepts contained within the Systems of Care (SOC) approach using a sample of 225 randomly selected counties stratified by population size and poverty level. Implementation was measured using the Systems of Care Implementation Survey (SOCIS), which was completed by 910 informants representing the major child serving systems (i.e., mental health, school, and other child agencies, such as child welfare and Juvenile Justice) and child mental health advocacy groups.In the article by Boothroyd, Greenbaum, and Wang et al., 1 the research design, instrument development, and pilot testing of the data collection strategy for assessing the level of SOC implementation is described. The results illustrate a strategy that would permit the collection of data on the level of implementation of children's systems of care principles from the perspectives of multiple stakeholders in a nationally representative sample of counties.The article by Greenbaum, Wang, and Boothroyd et al. 2 provides a detailed summary of the psychometric properties of the SOCIS, including documenting the factor structure, item performance, reliability, validity, and their association with different informant groups.