From the 17th to the late 20th century, doctors often acted upon anecdote, opinion, and tradition. As a result many useless and even some harmful treatments were employed. Evidence-based medicine is a late 20th century innovation. It is the “conscientious, explicit and judicious use of the current best evidence from clinical care research in making decisions about the care of individual patients.” It depends upon the accumulation of data from treating patients. This article describes the challenges and methods used in this process. Reviewed and updated 2009.