The Dialysis Outcomes and Practice Patterns Study (DOPPS) is a prospective, observational study designed to elucidate aspects of hemodialysis practice that are associated with the best outcomes for hemodialysis patients. In DOPPS I, 308 hemodialysis units from 7 countries participated, including 145 facilities from the United States (1996-2001), 62 facilities from Japan (1999-2001), and 101 facilities from France, Germany, Italy, Spain, and the United Kingdom (all 1998-2000). DOPPS II (2002-2004) has included 320 hemodialysis units and more than 12,400 hemodialysis patients from the 7 DOPPS I countries as well as Australia, Belgium, Canada, New Zealand, and Sweden. Dialysis units are chosen via a stratified random selection procedure to provide proportional sampling by region and type of facility within each country. In DOPPS I and II, longitudinal data have been collected from both a prevalent (cross-sectional) patient sample and an incident patient sample. Data have also been collected on numerous facility practice patterns. Most DOPPS analyses incorporate both facility- and patient-level data in regression-based analyses to investigate predictors of survival, hospitalization, quality of life, vascular access type, and other outcomes. DOPPS longitudinal data also help identify trends in subject characteristics, practice indicators, medication use, and outcomes. The DOPPS remains a unique source of data on hemodialysis patients and facilities. It continues to refine its methods of data collection and analysis with the goal of improving hemodialysis practice and end-stage renal disease patient lives worldwide.