Although well-designed randomized controlled trials (RCT) provide the strongest evidence regarding causation, only relatively recently have they been used by health services researchers to study the organization, delivery, quality, and outcomes of care. More recent yet is the extension of multisite RCTs to health services research. Such studies offer numerous methodological advantages over single-site trials: (1) enhanced external validity; (2) greater statistical power when studying conditions with a low incidence or prevalence, small event rate in the outcome (eg, mortality), and/or large variance in the outcome (eg, health care costs); and (3) rapid recruitment to provide health care organizations and policy makers with timely results. This paper begins by outlining the advantages of multisite RCTs over single-site trials. It then discusses both scientific challenges (ie, standardizing eligibility criteria, defining and standardizing the intervention, defining usual care, standardizing the data collection protocol, blinded outcome assessment, data management and analysis, measuring health care costs) and operational issues (ie, site selection, randomization procedures, patient accrual, maintaining enthusiasm, oversight) posed by multisite RCTs in health services research. Recommendations are offered to health services researchers interested in conducting such studies.