What is the evidence? Evidence-based medicine, or EBM which slips off the tongue more readily, seeks to offer proof that what we offer patients whenever we treat them, do a diagnostic test, or recommend anything from an operation to change in life style, is known to be effective. Fifty to 100 years ago, health care was delivered patchily, much less of it was effective, and was based on an image of a kindly doctor treating the sick, who were glad of whatever help could be offered. Now, medicine is effective and whether you look at it as major government expense or big business in private health care insurance, there is no sense in spending money on ineffective treatments, while we do not have enough to spend on what is of proven worth. Individual doctors cannot read everything and cannot possibly synthesise what they do read, so that they are aware of what is proven to be effective and what probably is not. The "The Evidence for ..." series seeks to fill that need. Into "The Evidence for Cardiothoracic Surgery" we have commissioned systematic reviews of the common operations and some innovative treatments, to present the facts to busy doctors. Where is there and where is there not evidence?