Linda Resnik1, Janet K Freburger2. 1. L. Resnik, PT, PhD, is a guest co-editor of this special series. She is director, Center on Health Services Training and Research (CoHSTAR); professor (research), Department of Health Services, Policy and Practice, School of Public Health Brown University; and Research Career Scientist, Providence VA Medical Center, Providence, Rhode Island. 2. J.K. Freburger, PT, PhD, is a guest co-editor of this special series. She is associate director and Senior Research Fellow, Cecil G. Sheps Center for Health Services Research, University of North Carolina at Chapel Hill.