Contributions of electron microscopy to our knowledge of fine kidney structure have been reviewed and extended. Notable findings include the following: 1 . The glomerular capillary is extraordinarily specialized, presumably to facilitate diffusion. Its epithelial cells are highly branched, and have vast numbers of terminal processes that interdigitate with one another, and with those of neighboring cells, over the surface of the capillary. Substantial diffusion spaces exist between processes, so this layer would not behave as a physiological barrier unless, possibly, the epithelial cells are contractile. Endothelial cytoplasm is in the form of a very attenuated sheet except in the vicinity of perikarya. The thin sheet, furthermore, is perforated by a closely-spaced system of holes or pores which average a little over 0.1 µ in diameter, and which would allow the unimpeded passage of even the largest molecules. The basement membrane can be divided into outer and inner "cement" layers of low electron density, and a middle structural layer of high electron density. These layers appear homogeneous even with good resolution. The basement membrane provides the only continuous barrier between vascular and urinary spaces. 2. The endothelium of the peritubular capillaries is greatly attenuated, and is perforated by holes much as is the glomerular endothelium. However, the pores are smaller (about 0.6 µ), and are more widely spaced. In the medulla only patches of fenestrated endothelium are found. The capillary basement membrane can be subdivided into a "cement" layer, and an excessively thin, dense, structural layer. This appears homogeneous, and must represent the only effective diffusion barrier between connective tissue spaces and the blood channels. 3. In the proximal tubule there is a system of vacuoles and intracellular canaliculi which open into the lumen between brush processes. The system apparently is quite labile, appearing most conspicuously in cells showing evidences of dehydration. This might constitute an accessory excretory mechanism, or possibly it represents an absorption route. 4. In the proximal, and particularly in the distal, tubule the plasma membrane at the basal margins of the cells is elaborately folded and intussuscepted deeply into the cytoplasm. Adjacent folds anastomose with one another in a manner which vastly increases the potential surface area of this end of the cell. The open-ended cytoplasmic compartments formed in this way contain most of the mitochondria of the cells. This system is also to be seen moderately developed in the collecting tubules, but not penetrating deeply enough here to incorporate the mitochondria. In the attenuated cytoplasm of the cells of the thin segment, the folds of the basal plasma membrane penetrate completely through to the lumen, and thus establish a system of slots through the cytoplasm of these cells. At the luminal surface, the slots have morphological features resembling terminal bars. It is possible that this system facilitates ionic transport. 5. Swedish investigators have found the proximal tubule a particularly favorable place to study mitochondria. Normal mitochondrial structure has been described in detail, and changes associated with protein resorption have been followed. 6. It has been observed that granules, thought to be made up largely of ribonucleic acid, are notably abundant in the cytoplasm of the proximal tubule. They are not conspicuously numerous in other parts of the nephron.