Health services researchers have long been aware of large variations in the use of medical care across regions and medical providers. In the 1930s, the British pediatrician J.A. Glover observed that the rates of tonsillectomy in British schoolchildren varied widely, depending on the district where the students lived and the doctors who examined them. This volume provides a contextual landscape for the study of health care utilization through the lens of medical practice variations. It is grounded in the pioneering work by medical care epidemiologist, Dr. John Wennberg, who revealed wide variations in elective surgical rates across small areas in the U.S. and his findings that these variations were generally not explained by differences in population illness rates or patient preferences but rather, there were strong associations between supply of health care resources, such as hospital beds and physicians and health care utilization. This volume introduces the concept of medical practice variations and its early history, outlines established concepts and frameworks, with an overview of methods used to understand the variations in medical care. It makes the case for outcomes research in determining what works in health care and policy reforms to rationalize how care is delivered. Each chapter synthesizes the current published literature in the field and covers a description of medical practice variations in the area, determinants of these variations and outcomes. It outlines the most current research on specific types of utilization such as inpatient care, emergency services, elective surgery, primary care, obstetric and gynaecological care, mental health care and end-of-life care, among others. Studies of variation in condition-specific care focus on common conditions such as acute myocardial infarction, congestive heart failure, stroke, diabetes and procedures such as cancer surgery and joint replacement. Special topics include health care spending and quality, shared decision making and disparities.