Hydronephrosis is defined as dilatation of the drainage system of the kidney, i.e., the calices, the infundibula, and the pelvis. Although this term etymologically does not express the pathological condition per se, it is so universally accepted that its elimination might cause considerable confusion. In spite of this fact, we should like to recall the suggestion made by others in favor of the more descriptive designations “pyelectasis” and “caliectasis.” In this paper, however, we shall mainly use the traditional term. From the point of view of their mechanism, hydronephroses comprise two basic etiologic groups, according to whether or not they are caused by an obstruction. Where they are so produced, the obstruction creates a hindrance to the free flow of urine. This in turn leads to back pressure, dilatation, parenchymal damage, and impairment of function, with ultimate complete loss of renal tissue. The hydronephroses without demonstrable obstruction (Figs. 1 and 2) include cases of congenital origin (analogous to Hirschsprung's disease), infection, and to some extent the important group of pregnancy hydronephrosis. There is much controversy as to the pathogenesis of the non-obstructive hydronephroses; various theoretical speculations account for them on the basis of toxic, hormonal, neurogenic, or neuromuscular factors. The great majority of hydronephroses can be identified as caused by obstruction, and therefore the present discussion is mainly based on that group. The obstruction causing hydronephrosis may be due to either an intrinsic or an extrinsic cause, each of which in turn may be either congenital or acquired. Intrinsic Causes Congenital Polycystic kidney Tumor Stone Kink Ureteral fold Ureteral valve Ureteral stricture Ureterocele Contracted vesical neck Urethral valve Urethral stricture Acquired Stone Tumor Ureteral stricture Inflammation, infection (a) non-tuberculous (b) tuberculous Contracted vesical neck Urethral stricture Cyst and hypertrophy of verumontanum Extrinsic Causes Congenital Fused kidney; horseshoe kidney; cake kidney Aberrant vessel Abnormal insertion of ureter Abnormal rotation of kidney Ectopic kidney Double pelvis Double ureter Aneurysm of renal artery Tumor compressing urinary tract Acquired Infection Adhesions, fibrous bands Tumor compressing urinary tract Dystopic kidney Ptosis Scoliosis Trauma (surgical or other injury) Hypertrophy of prostate gland The obstructive process may occur at any level of the urinary tract. Early recognition of the condition and of the cause and site of the obstruction is essential for successful treatment and for prevention of permanent damage to the kidney (Figs. 3 and 4). The character of the dilatation of the pelvis, calices, or both, depends strictly on the type of the pelvis.