Abstract Tetralogy of Fallot is the most common form of cyanotic congenital heart disease and one of the first to be successfully repaired by cardiac surgeons specializing in congenital defects Since the first procedures in the 1950s, advances in the diagnosis, perioperative and surgical treatment, and postoperative care of tetralogy of Fallot have been such that almost all those born with this condition can now expect to survive to adulthood This chapter focuses on new developments in the understanding of the causes, diagnosis, early treatment, and late outcomes of tetralogy of Fallot Keywords Tetralogy of fallot; pediatric cardiology; cardiac surgery; interventional catheterization Introduction Lev and Eckner1 made the appropriate statement that no two cases of tetralogy are exactly the same Although that is fundamentally true, the characteristic anatomy permits its instant recognition as a phenotypic entity It is appropriate that we continue to use the term named in honor of Arthur Louis Etienne Fallot, who described the combination of an interventricular communication, biventricular origin of the aorta, muscular obstruction within the right ventricular outflow tract, and right ventricular hypertrophy (Fig 35.1, left).2 FIG 35.1 Left, Images taken from the apex of the morphologically right ventricle looking toward the base showing the phenotypic features of tetralogy of Fallot Right, The major phenotypic feature, insertion of the muscular outlet septum, that separates the overriding component of the artic outlet from the subpulmonary (sub-pulm.) infundibulum to the anterocephalad limb of the septomarginal trabeculation (yellow bars) is shown As can be seen, this produces a squeeze at the mouth of the infundibulum between the deviated outlet septum and the septoparietal trabeculations (yellow bracket) Hearts with this phenotypic morphology had been described prior to Fallot's description His contribution was to note that the combination of lesions accounted for the majority of cases of “la maladie bleu,” or cyanosis, seen at autopsy Maud Abbott introduced the concept of a tetralogy and its eponym, which has stood the passage of time.2 Arguments continue, however, regarding the essence of the malformation and which variants should appropriately be described within the eponym When obstruction within the right ventricular outflow tract is minimal, it can be hard to distinguish tetralogy of Fallot (TOF) from the variant of ventricular septal defect with aortic overriding, sometimes described as the Eisenmenger defect.3 When obstruction is complete, the condition represents the most common variant of pulmonary atresia with ventricular septal defect (correctly described as TOF with pulmonary atresia under such circumstances) The single feature underscoring the phenotypic findings is anterior and cephalad deviation of the septal insertion of the outlet septum relative to the septomarginal trabeculation, usually combined with abnormal septoparietal trabeculations (Fig 35.1, right) Within the hearts fulfilling this criterion, there is ample room for significant individual variation.4 This variation is the focus of anatomic introduction to this chapter, placing emphasis on the major differences from the normal heart