This is an editorial review of investigations into the correlation of structure and function of the kidney in various inflammatory and noninflammatory glomerular diseases and in focal and diffuse interstitial nephritis. In detail these investigations produced the following results:(1) The excretory function of the glomeruli for substances usually eliminated with the urine is, in the case of inflammatory and noninflammatory glomerular diseases, detrimentally affected by tubulointerstitial changes, ie by processes accompanied by interstitial fibrosis and tubular atrophy. Likewise primary interstitial renal diseases when accompanied by interstitial fibrosis and tubular atrophy may lead to reduction in GFR.(2) Inflammatory and noninflammatory glomerular diseases, even when very severe, are not accompanied by a measurable reduction in GFR when the renal cortex interstitium shows no changes and the tubules exhibit no pathological findings.(3) The concentration ability of the kidney, too, depends primarily on tubulointerstitial changes and not primarily on a reduction of the glomerular filtration surface area. As interstitial fibrosis and tubular atrophy increase, the maximum concentration ability of the kidney decreases, even when the glomerular structure is preserved.(4) The decrease in GFR in the case of processes in the renal cortex accompanied by severe interstitial fibrosis is the result of the reduction of the number and of the area of the postglomerular vessels, ie the result of an impeded outflow from the glomeruli and of a concomitant slower circulation through the glomeruli.(5) In the case of inflammatory and noninflammatory glomerular and extraglomerular renal diseases accompanied by slight interstitial fibrosis and tubular atrophy, the GFR is detrimentally affected via a hormonally controlled self-regulating mechanism (Thurau-mechanism) in the form as modified by Baumbach and Skott and Leyssac. The glomerular function thereby adapts to an insufficient tubular function, without there necessarily being any structural changes in the glomeruli.