Childhood chronic illnesses affect between 7% and 18% of children, and each of these illnesses requires decisions concerning the timing, completion, and monitoring of complex treatment regimens. The process by which children achieve greater independence for decision making related to these regimens may influence effective self-management and health outcomes. Collaborative decision making (CDM) between parents and children is a potentially important precursor to full decision making independence during childhood and adolescence. CDM provides an opportunity for children to learn what family members take into account when making decisions and the consequences of different decisions, which may contribute to the development of the child's own decision making skills. Because of its potential role in the successful transition to independent and effective decision making, additional empirical data related to collaborative decision making for illness management is needed. The primary objective of this research is to determine developmental mechanisms, predictors, and outcomes of collaborative decision making in children and adolescents with a chronic illness (type 1 diabetes or cystic fibrosis) and their parents. This research addresses the following specific aims: 1) To describe developmental trajectories of CDM and to identify the developmental mechanisms underlying these trajectories (e.g., cognitive development; psychosocial maturity), 2) To identify characteristics of the child, parent, and family that are related to patterns of CDM over time (e.g., child health locus of control; parent autonomy support; family communication), and 3) To determine if CDM is related to patterns of treatment adherence and responsibility over time. The methods incorporate an accelerated longitudinal design, in which we assess multiple age cohorts at Time 1 and then collect longitudinal data on members of each cohort over a three-year period. This approach minimizes the disadvantages of the traditional longitudinal design and allows us to examine a broad range of development (age 8-18 years) in a shorter study period than would otherwise be possible. Very few studies have utilized longitudinal methodology to understand chronic illness management as a function of developmental variables. This research has significant implications for the health and well-being of children and adolescents with a chronic illness, by seeking to understand how they learn to manage their health both independently and effectively. Furthermore, the transition to greater independence may provide the foundation for effective disease management during adulthood, making it a critical area of empirical investigation. Data from this project will lead to the development of intervention strategies to enhance parent-child decision making interactions about chronic illness management at different ages. This data will also enable us to identify early patterns of interacting that either facilitate or impede adherence and responsibility later on, so that clinicians can intervene before problems develop.