The aim of the Aristotle project was to develop a new method of evaluation of quality of care in congenital heart surgery based on the complexity of the surgical procedures. Involving a panel of expert surgeons, the project started in 1999 and included 50 pediatric surgeons from 23 countries representing International Scientific Societies. The complexity was based on the procedures as defined by the Society of Thoracic Surgeons (STS)/European Association for Cadiothoracic Surgery (EACTS) International Nomenclature and was undertaken in two steps: The first step was establishing the Basic Score, which adjusts only the complexity of the procedures and is based on three factors: the potential for mortality, the potential for morbidity, and the anticipated technical difficulty. The second step was the development of the Comprehensive Score, which further adjusts the complexity according to the specific patient characteristics. The Aristotle score allows precise scoring of the complexity for 145 congenital heart surgery procedures. One interesting concept coming out of this study is that complexity is a constant and precise value for a given patient regardless of the center where he is operated. The Aristotle method allows proposing the following equation of quality of care: Complexity FN Outcome = Performance. The Aristotle score, electronically available, was introduced in the EACTS and STS databases. A validation process, designed to evaluate its predictive value, is being developed.