The morphological structure of the kidney only started being seriously investigated and truly understood in the 16th century, thanks mainly to Bartolomeo Eustachio. In his De Renibus, in fact, he painstakingly described the size, consistency, location and variations of the kidney, to which modern understanding can add very little. In describing the renal parenchyma he stated that it is made up of an external and an internal substance, and recognized the central role of the renal arteries in the excretory function of the kidneys. He made observations regarding the presence of extremely fine arteries that filter urine, the nature and function of the renal tubules and the columns of external substance that protrude between the papillae. There can be no doubt that Eustachio’s remarkable achievements made him a pioneer in morphological studies of the kidney.