Background <p>“An expert is an ordinary fellow from another town.” (Mark Twain)</p> <p>Expert evidence in medical claims will be required to commence almost any case involving professional negligence, and as modern medical practice, in all of its guises, becomes increasingly more litigious, expert medical witnesses are required more frequently. Expert evidence is an essential element of advancing, or defending, a clinical negligence claim. There is a lack of guidance for physicians in this jurisdiction in how to prepare and act as an expert witness, to such an extent that the Medical Protection Society (MPS) has called for a wider pool of expert medical witnesses in the Irish courts system (1). This follows a recent warning from Mr. Justice Collins in a Court of Appeal judgement (2) that a significant change of culture on expert evidence was needed. The MPS has called on the Health Service Executive to support doctors in undertaking expert-witness training and to maintain a central list of experts (1).</p> Aims and methods <p>The aim of this article is to provide a comprehensive understanding of the role and duties of the expert witness, by using guidance from various jurisdictions, while also discussing the penalties that may ensue if they are not fulfilled.</p> Conclusion <p>Flawed evidence can lead to miscarriages of justice (3) and, in turn, to a lack of confidence in justice and a degradation of the rule of law. This is why expert evidence is absolutely fundamental to the rule of law, as flawed expert evidence can lead to a court acting in good faith but reaching an unsound decision. Physicians serving as expert witnesses must therefore fully grasp the scope and significance of their role—not only to uphold the law and maintain the integrity of their profession but, most importantly, to fulfil their duty to patients.</p>