Electron microscopy was first applied to the study of the kidney some 25yr ago. In the early 1950's the normal ultrastructure was established [1, 2] providing a basis for the studies of abnormal morphology. A large number of papers dealing with human and experimental renal disease appeared during the next 15yr and several comprehensive reviews were published in the late 1960's [3–6]. The high resolution of the electron microscope enabled the investigators to shed light on many problems which had caused controversy among the light microscopists, among them the structure of the capillary wall in the glomerulus, the nature of the mesangium and its behavior in disease and the nature and location of "fibrinoid" deposits in various glomerular diseases. Electron microscopic studies contributed significantly to the understanding of the pathogenesis and the morphogenesis of the disease process in the glomeruli and to a lesser extent in the tubules, the interstitial tissue and the blood vessels. It also proved to have definite diagnostic value. In this review only the more important contributions of the earlier periods will be discussed and will be supplemented by the more recent information covering approximately the last ten years.