Acute kidney injury (AKI) is often overlooked in hospitalized patients, despite the fact that even mild forms are strongly associated with poor clinical outcomes such as increased mortality, morbidity, cardiovascular failure and infections. Research endorsed by the Acute Dialysis Quality Initiative led to the publication of a consensus definition for AKI--the RIFLE criteria (Risk, Injury, Failure, Loss of function, and End-stage renal disease)--which was designed to standardize and classify renal dysfunction. These criteria, along with revised versions developed by the AKI Network (AKIN), can detect AKI with high sensitivity and high specificity and describe different severity levels that aim to predict the prognosis of affected patients. The RIFLE and AKIN criteria are easy to use in a variety of clinical and research settings, but have several limitations: both utilize an increase in serum creatinine level from a hypothetical baseline value and a decrease in urine output, but these surrogate markers of renal impairment manifest relatively late after injury has occurred and do not consider the nature or site of the kidney injury. New biomarkers for AKI have shown promise for early diagnosis and prediction of the prognosis of AKI. As more data become available, they could, in the future, be incorporated into improved definitions or criteria for AKI.