The current project leverages newly formed Regional Care Collaborative Organizations (RCCO) in Colorado that are charged with providing comprehensive care for children and adults insured by Medicaid. With a new RCCO-public health collaborative we will implement a randomized controlled trial to test the effectiveness of 1) evidence-based collaborative centralized method of increasing immunization delivery using reminder/recall techniques versus 2) traditional practice-based reminder/recall techniques that are done by individual practices (not centrally) or usual care from their provider. These interventions have been previously developed by the study team in different clinical settings. The project will examine the collaborative centralized approach, implemented within the new RCCO-public health collaborative first among preschool children, the group in which it has been previously studied. We will then extend the method to new populations, including adolescents and adults within the RCCO and Denver Health (adults only), and will evaluate the effectiveness of the intervention in each population.