The arterial switch operation has become a safe operation in many centres. The complexity of the procedure has evolved over the last two decades. Several anatomical features can hardly be considered complex today, namely, normal coronary anatomy, circumflex coming off the right coronary artery, eccentric ostium, and early take-off of an infundibular artery. In addition, as peri-operative mortality becomes very low, the outcomes need to be evaluated on the peri-operative morbidity, late reoperations, and late deaths. The arterial switch operation remains complex in around 20% of the cases, where one or several complexity factors are associated. The complexity of the coronary arteries is a major factor. According to a classification essentially based on the course of the coronary arteries, complex coronaries include: double-looping coronaries, anterior-looping coronaries, intramural coronaries, and single coronary ostium. The most challenging coronary pattern remains the association of a single ostium with intramural course. Other features are equally complex: severe malalignment of the commissures, aortic arch obstruction, multiple ventricular septal defect, Taussig-Bing with subaortic obstruction, double-outlet right ventricle non-committed ventricular septal defect, transposition of the great arteries-intact ventricular septum >3 weeks, transposition of the great arteries-ventricular septal defect with high lung resistances and weight <2.5 kg. Owing to the fact that the risks of arterial switch operation vary according to the experience of the centres, we defined the arterial switch operation complexity based on a subjective approach as proposed by the Aristotle comprehensive score. The recent introduction of a morbidity score will allow to stratify more accurately the outcomes when the peri-operative mortality is very low or nil. The complexity of the coronary patterns tends to be well controlled today. It remains that rare coronary failures and aortic root dilation will occur in the long term, requiring a close follow-up of the most complex patients. Successfully achieving complex arterial switch operation implies a second learning curve.