Evidence from various sources indicates that a substantial number of hospitalized patients suffer treatment-caused injuries. Most of these injuries result from errors. Yet physicians and other health professionals have been reluctant to admit and address the problem of errors, both because of feelings of guilt and from the desire to avoid punishment or disapproval by colleagues. Research in cognitive psychology and human factors has shown that most errors result from defects in the systems in which we work. These are failures in the design of processes, tasks, training, and conditions of work that make errors more likely. Meaningful reduction of errors requires correction of these systems failures. Barriers to reduction of errors include the complexity of health care systems, difficulties in information access, tolerance of stylistic practices, and fear of punishment that inhibits reporting. Most institutions also lack effective methods for detecting and quantifying errors. Significant improvements in error reduction will require major commitments by organizational leadership and the recognition that errors are evidence of deficiencies in systems, not deficiencies in people.