Congenital anomalies may involve any level of the collecting system; the most usual presentation is urinary tract dilatation that may already be detected during fetal life. The role of imaging is to determine the origin of the dilatation, i.e., obstructive versus nonobstructive (Table 1 lists the causes of urinary tract dilatation). Other useful information includes the level of the impairment to drainage (so-called obstruction) and its impact on renal function. All these data are important in order to determine the best therapeutic approach.