It was not until recently that consensus definitions for acute kidney injury (AKI) were proposed and published. The RIFLE (Risk-Injury-Failure-Loss- End-stage renal disease) and AKIN (Acute Kidney Injury Network) classifications were designed in order to be easily understood and applied in a variety of clinical and research settings. Their creation was intended to uniformly establish the presence or absence of the AKI and to give a quantitative idea of the severity of the disease unifying the commonly used parameters of serum creatinine and urine output. Subsequent validation showed that both the presence and severity of AKI, defined using RIFLE/AKIN, correlate well with patient outcome. This review will briefly describe the RIFLE/AKIN consensus definitions, its subsequent revisions and its successful validation and application to clinical research. The potential of extending the use of RIFLE/AKIN to the clinical setting of cardiorenal syndromes is also discussed.