Acute kidney injury (AKI), often used interchangeably with acute renal failure (ARF), is a rapid decline in the ability of the kidney to clear waste products. AKI is a common postoperative complication that increases morbidity and mortality, especially after major surgical procedures, including complex cardiac, vascular, and major abdominal surgery. The standardized criteria for diagnosing AKI were established by the risk, injury, failure, loss, and end-stage kidney disease (RIFLE) criteria in 2004 and were modified to the Acute Kidney Injury Network (AKIN) criteria in 2007 using the serum creatinine level and urine output as indicators of kidney function. The most recent consensus statement on the diagnosis of AKI is the 2012 Kidney Disease: Improving Global Outcomes (KDIGO) criteria (Table 131.1) which classify AKI into stages based on severity. The higher stages are associated with worse outcomes, mortality, and need for renal replacement therapy (RRT). Acute kidney injury (AKI) is a common postoperative complication. An understanding of the definitions, etiologies, diagnosis, and treatment of AKI is critical to successful treatment of the surgical patient. Serum creatinine may vary with age, muscle mass, and diet. Knowing a patient’s baseline renal function can be important in managing the patient postoperatively. Management of AKI is based upon likely cause(s)—generally prerenal, intrinsic renal, or postrenal etiologies. Indications for, timing of, and modalities of renal replacement therapy may impact morbidity and mortality.