<jats:p> Individual patient randomized trials are the gold standard for assessing the effects of health care evaluations. However, individual randomization may not be possible for practical, logistical, ethical, or political reasons, for example, when evaluating health care professional and organizational behavior change interventions. Under such circumstances, cluster randomized trials are commonly used. This article discusses the practical and ethical issues in the design, conduct, and analysis of cluster randomized trials of professional behavior and organizational change strategies using examples from two primary studies evaluating health care provider behavior change strategies. Cluster randomized trials are commonly used in health care. They raise distinct ethical and methodological issues that have rarely been adequately addressed in studies to date. </jats:p>