It appears difficult to build a coherent picture of the concentrating system of the mammalian renal medulla. This may be due to the diversity of the factors involved and to considerable interspecies differences. Several morphological adaptations that may be critical in the improvement of water conservation are described. They include variations in the length of the papilla, number of nephrons, percentage of long-looped nephrons, nephron heterogeneity, development of pelvic fornices, confluence of collecting ducts, vascular bundles in the inner stripe of the outer medulla, thin descending limb epithelium, and relative development of the three medullary zones. The organization of the medullary circulation is described; the medulla includes several functionally different compartments favoring preferential exchanges by the juxtaposition of certain tubules and vessels. This may improve the efficiency of certain recycling routes and hence the insulation of the different compartments. As discussed in section III, a better inner medullary insulation may be key, not (or not only) in achieving a high urine concentration but mainly in reducing the time required to reach this high concentration. This overview of the multiple interspecies variations in medullary organization underlines the importance, among other factors, of the inner stripe architecture and of the internephron differences in the process of urine concentration.