The purposes of this article are to (a) describe the design features of two longitudinal studies, the Special Education Elementary Longitudinal Study (SEELS) and the National Longitudinal Transition Study-2 (NLTS2) and (b) outline their potential implications for policy, practice, research, advocacy, and system development for children and youth with emotional disturbances (ED). We begin by reviewing the role of longitudinal research and its importance in building the knowledge base regarding these children and youth. A review of illustrative longitudinal studies conducted on children and youth with ED points to the potential of a longitudinal design for providing much needed information but also reveals the limitations and inconsistencies in both the samples and methods of existing studies that hamper development of policy and practice. SEELS and NLTS2 are offered as exemplars of the type of studies needed to supply critical information to the field. We describe key features of these studies and how they will extend the research knowledge base. We conclude by outlining the implications of the two studies for key issues in improving outcomes for children and youth with ED.