The toll of inadequate health care is well-substantiated, but recognition is mounting that "too much" is also possible. Overdiagnosis represents one harm of too much medicine, but the concept can be confusing: It is often conflated with related harms (such as overtreatment, misclassification, false-positive results, and overdetection) and is difficult to measure because it cannot be directly observed. Because the U.S. Preventive Services Task Force (USPSTF) issues screening recommendations aimed largely at healthy persons, it has a particular interest in understanding harms related to screening, especially but not limited to overdiagnosis. In support of the USPSTF, the authors summarize the knowledge and provide guidance on defining, estimating, and communicating overdiagnosis in cancer screening. To improve consistency, thinking, and reporting about overdiagnosis, they suggest a specific definition. The authors articulate how variation in estimates of overdiagnosis can arise, identify approaches to estimating overdiagnosis, and describe best practices for communicating the potential for harm due to overdiagnosis.