The purpose of this Funding Opportunity Announcement (FOA) is to encourage grant applications for investigator-initiated prospective observational comparative effectiveness research (CER) to the National Institute of Neurological Disorders and Stroke (NINDS). The study must address questions within the mission and research interests of the NINDS and may evaluate preventive strategies, diagnostic approaches, or interventions including drugs, biologics, and devices, or surgical, behavioral, and rehabilitation therapies. Information about the mission and research interests of the NINDS can be found at the NINDS website (https://www.ninds.nih.gov/). Studies proposed should provide a cost-effective means of collecting data with a meaningful bearing on current clinical practice. Awards made under this FOA will initially support a milestone-driven planning phase (UG3) for up to 2 years, with possible transition to a observational study phase of up to five years (UH3). Only UG3 projects that have met the scientific milestones and feasibility requirements may transition to the UH3 phase. The UG3/UH3 application must be submitted as a single application, following the instructions described in this FOA. The UG3 phase for observational studies will permit both scientific and operational planning activities. Scientific planning activities include small-scale data collection to assess the feasibility and/or acceptability of data collection, storage, and planned analyses. Operational planning activities include, at a minimum, development of recruitment and retention strategies, case report forms, data management system and other tools for data and quality management. The UH3 phase of the award will support the conduct of investigator-initiated observational study.