Clinical practice guidelines can be great. It’s nice to have guidelines for the diagnosis, management, and long-term care objectives for basically every condition known to medicine. Clinical practice guidelines are supposed to be the foundation of 21st century evidence-based medicine. But clinicians frequently do not follow clinical practice guidelines. Just Google “clinical practice guidelines”: after you note 7,100,000 citations on clinical practice guidelines in less than 0.5 seconds, you begin to understand why. First, which clinical practice guideline do you follow? Various professional organizations fight over the same meta-analyses and come to different conclusions—notoriously in regard to mammography recommendations, for example. There are differences in the United States among different organizations regarding when to start mammography, how often to screen, and when to stop. Other advanced medical systems in Europe look at the same data and choose very different intervals and start-up and ending ages. There are distinguished researchers who maintain that screening mammography in low-risk patients is entirely without merit and may do more harm than good.1 Which clinical practice guideline do you follow and how did you choose it? Or choose to ignore it?