To assess, plan, and implement care of the critically ill neonate, the nurse must have a thorough understanding of renal embryology and physiology. This articles focuses on the developmental and anatomical structure of the kidney and the general physiological activities of the renal components. Renal embryological development begins during the 1st week of gestation and continues until around the 36th week of gestation. Functional capacity, although not mature, begins around the 6th week of gestation. Infants born premature have underdeveloped structures and decreased renal function. Comprehending renal anatomy and physiology requires a complete understanding of a single nephron, the functional unit of the kidney. Each nephron consists of a glomerulus, Bowman's capsule, and tubules, which work together to maintain ion balance for cellular function and elimination of unwanted substances from the plasma. Renal blood flow, controlled by either autoregulation or hormonal control, must remain both rapid and constant for glomerular filtration to occur. Alterations in any of these components of renal anatomy and physiology will alter the condition of the neonate. Renal evaluations are done based on output, urine chemistries, and serum chemistries. These evaluations lead to a diagnosis on which the nurse can base her plan of care.