The renal system is composed collectively of the kidneys, ureters, urinary bladder and urethra. Together, the kidneys receive approximately one-quarter of the body's cardiac output, which is necessary to perform their primary function of filtration. However, the renal system - or, more specifically, the kidneys - performs seven important homeostatic functions: 1 regulation of fluid, electrolyte and calcium balance 2 regulation of acid-base balance 3 excretion of metabolic wastes 4 regulation of blood pressure 5 regulation of vitamin D production 6 regulation of red blood cell production 7 gluconeogenesis. Acute kidney injury (AKI) can occur to anyone within the community or hospital setting, and AKI not only causes interruption to the seven homeostatic functions of the kidneys but can also impact other major body organs. This chapter considers the pathophysiology of the kidneys, causes, types and clinical features of AKI and, crucially, the nurse's role in the prevention and early detection of this life-threatening illness.