Chronic pyelonephritis (c.p.) is by definition an infectious tubulo-interstitial nephritis. It has to be differentiated from other etiologic forms of tubulo-interstitial nephritis. Therefore strict morphological criteria are needed for diagnosis. The characteristic lesion is a large cortico-medullary scar overlying a dilated chronically inflammed calyx. The macroscopic aspect and the histologic survey picture are more important than histologic details. A diagnosis on renal biopsies is therefore not warranted. Vesico-renal reflux and papillary morphology play an important pathogenetic role. Beside the more common focal scar a diffuse form of scarring can be observed. A limited number of conditions only have to be considered in differential diagnosis. The Ask-Upmark kidney seems to be a special form of c.p. related to urinary tract infection and reflux in early infancy. Pelvi-calyceal lithiasis without superimposed infection causes a picture very similar to a pyelonephritic scar. A reliable differentiation between c.p. and analgesic nephropathy may cause problems in endstage kidneys with sloughed off papillae. Various mechanisms of renal damage such as bacterial infection, immunological mediated inflammation, leakage of urinary constituents into the interstitium especially Tamm-Horsfall-protein and ischemia have to be considered. Despite the frequency of urinary tract infections chronic progressive pyelonephritis is rare. Predisposing factors are needed for progression of the disease. These include congenital or acquired urinary tract obstruction, vesico-renal reflux and papillary damage with intrarenal obstruction to the urinary flow. Other important factors are focal and segmental glomerulosclerosis and hypertension.