This is a prospective, non-randomized, single center study designed to measure and determine the changes in cerebral blood flow in a cohort of single ventricle patients followed across all three stages of surgical (Fontan) reconstruction. In addition, cerebral blood flow response to hypercarbia will be determined after the first 2 operations as a measure of cerebral blood flow reserve and the "intactness" of the cerebral autoregulatory system. MRI data from this cohort will be compared with that collected in age-matched healthy children and in a cross sectional group of patients who will not necessarily be followed through all stages of surgery, but may undergo one or two of the MRI evaluations depending on their stage of surgery. Neurological exams and EEGs will be performed. Medical history, cardiac catheterization data, where available, will also be collected. Factors such as cardiopulmonary bypass time and socioeconomic class will be controlled for.