Public and private purchasers and health plans are demanding more information about the quality and relative costliness of U.S. physicians to increase physician accountability and aid in value-based purchasing. Although performance measurement has been in place for some time in hospitals and managed care organizations (MCOs), the focus on physician profiling is a relatively new development. The inherent limitations of the available data at the physician level have brought to the fore technical issues that were less important at higher levels of aggregation in hospitals and MCOs. One of these technical issues is the reliability of a physician's performance measurement. This technical report explains the use and implementation of reliability measurement for quality measures in provider profiling in health care. It provides details and a practical method of how to calculate reliability measures from the sort of data typically available. It also explains why reliability measurement is an important component of evaluating a profiling system.