With and increasing number of early survivors after the palliation of the single ventricle physiology there is a burgeoning Fontan population worldwide that will pose unique challenges because of the inevitable sequelae related to the absence of the alleged "needless" sub-pulmonic ventricle. The increasing number and older-age single-ventricle patients highlights the results of successful contemporary surgical palliation in children, leading to the development of an adult single-ventricle population with unpredictable socio-economic and health service impacts. The wide variability in clinical status of patients with Fontan circulation reflects not only the broadened spectrum of morphological substrates involved, but also the evolving surgical techniques during the last four decades. This has come in the wake of a gradual understanding of an incredibly tricky physiology. The magnitude of the disease, the physio-pathological mechanisms, and the therapeutic options to optimize the "failing Fontan" status and to delay the irreversible deterioration of "Fontan failure" condition are discussed in this review.