Traditionally, the etiology of acute kidney injury (AKI) is considered in terms of prerenal, renal, and obstructive causes. However, this categorization is less useful in the ICU, where the etiology of AKI is usually multifactorial and often occurs in the context of multi-organ failure. Hypotension, nephrotoxic drugs, and severe sepsis or septic shock are the most important identifiable factors. Less frequently encountered causes include pancreatitis, abdominal compartment syndrome, and rhabdomyolysis. Primary intrinsic renal disease such as glomerulonephritis is extremely uncommon. A previous history of cirrhosis, cardiac failure, or haematological malignancy, and age >65 years, are important risk factors. This chapter covers symptoms, complications, diagnosis, investigations, prognosis, and treatment of renal failure in the ITU.