Acute renal failure (also referred to as acute kidney injury) refers to a rapid decrease in renal function; it is reflected by an increase in blood urea and creatinine and is often associated with oliguria (a urine volume of less than 400 ml/24 hours). It usually develops over days to weeks. Acute kidney injury has been variously classified, but the current classifications are based on the glomerular filtration rate (or creatinine), looking at changes from baseline, and the presence of oliguria or anuria. The potential etiologies of acute kidney injury are usually considered anatomically under the headings prerenal, renal (intrinsic), and postrenal. This chapter looks at the etiology, symptoms, clinical features, demographics, complications, diagnosis, and treatment of acute kidney injury.