Professor Marc de Leval (16 April 1941 –26 June 2022) was a pioneer in the field of congenital cardiothoracic surgery and former chair of the British Congenital Cardiac Association [https://www.bcca-uk.org/]. Professor de Leval was appointed as a Consultant Cardiothoracic Surgeon at Great Ormond Street Hospital in 1974. Throughout his distinguished career at Great Ormond Street Hospital, he worked with dedication to improve outcomes for cardiothoracic surgery. His contribution to academic cardiothoracic surgery was significant and far-reaching with over 300 peer-reviewed publications. Of particular note was his work in improving the palliation of children with cyanotic congenital heart disease by the use of the modified Blalock-Thomas-Taussig shunt and his pioneering work over many years to improve understanding of the Fontan circulation using in vitro modelling and computational fluid dynamics that led to the development of the total cavopulmonary connection. His other significant contributions include the importance of analysing surgical failures, being the co-editor of the most readable textbook in our field, and so much more. During his career, he trained many eminent surgeons from around the world, and a Marc de Leval Fellowship of The American Association for Thoracic Surgery [https://www.aats.org/about-the-foundation/the-reach-of-our-programs/foundation-honoring-our-mentors-program/marc-r-de-leval-md] now allows the opportunity for a trainee to study congenital cardiac surgery in the UK or Europe. Marc is fondly remembered by the staff at Great Ormond Street Hospital. The nurses recall his arrival at the break of day on the intensive care unit, having already been for a swim, and the diligence with which he observed his patients following surgery. Former trainees remember a man who, despite his standing, always remained courteous, approachable, and kind. Professor de Leval will be remembered not only as a skilled and dedicated surgeon but also as a distinguished scholar, one with a quest for knowledge in the pursuit of excellence.