The proliferation of practice guidelines and recent controversies about cancer screening guidelines highlight the need to decide which guidelines are trustworthy. Cancer screening guidelines exemplify the challenge of public trust in guidelines. A firestorm of controversy (created in part by news media, professional organizations, disease advocacy groups, and politicians) surrounds discussions of screening for prostate cancer (should screening be routinely recommended, discussed, or discouraged?), breast cancer (should screening start at age 40 or 50 years?), and colon cancer (is colonoscopy preferred or are any of several test strategies acceptable?). Trust is important because guidelines set the de facto standard for medical practice and therefore influence clinical decisions about individual patients, practice measures, insurance coverage, and reimbursement.