Ultrastructure in Biological Systems, Volume 2: Ultrastructure of the Kidney provides an overview of the state of knowledge on the ultrastructure of the mammalian kidney. The application of the electron microscope to studies of the kidney resulted in the demonstration of the hitherto undetected early thickening of the basement membrane of glomerular capillaries in glomerulonephritis. Yet many problems remain, particularly in relation to the correlation between function and the ultrastructure of components of the kidney—mesangium, glomerulus, juxtaglomerular apparatus, and the renal tubules. It is only recently that the mesangium has come to be accepted as real, and many questions remain as to the function of its cells. The existence of true membranes between foot processes of the epithelial cells of glomeruli is a newly established fact; but what this has to do with glomerular filtration is not known at present. Granules apparently secretory in nature have been identified in cells of the juxtaglomerular apparatus, but so far their presence has not been correlated with specific functional change. Artifacts introduced at fixation are now known to have considerable relevance in interpreting the ultrastructure of the normal nephron. These are paraphrased views of the contributors to this monograph who, while acquainting the reader with the research being carried on in these areas, have also brought into focus the many problems still awaiting solution.