The contribution of electron and immunofluorescence microscopy to renal biopsy diagnosis is illustrated by the results obtained in a personal series of patients with various types of glomerulonephritis. Introductory notes on the ultrastructure of the glomerular capillary and on immunological processes are also included. Immunofluorescent staining has particular value in demonstrating IgG-containing deposits in early membranous glomerulonephritis at a stage when ordinary microscopy is inconclusive. It is capable of throwing light on the mechanism of glomerular damage in severe extracapillary proliferation and in some cases of recurrent haematuria, but is less successful in separating minimal change disease from proliferative processes. Electron microscopy reveals the precise site of immune deposits and fibrin together with basement membrane changes, the microtubular structures common in SLE, and other details. It is concluded that for the accurate diagnosis of kidney disease it is essential to supplement light microscopy by one, or preferably both these methods.