The origins of standards and guidelines in medicine are traced from work in the 1970s showing that treatment variations could not be accounted for by objective differences in the disease or other conditions of patients. Guidelines based on evidence can be effective in reducing such variation and in reducing costs. However, population disparities in disease and access, as well as dramatically rising insurance costs, pose challenges. Standards based entirely on efficacy of procedures will leave unanswered important questions about diagnosis and effective allocation of resources throughout population groups.