The understanding of acute kidney injury (AKI) has evolved from the perception of a single disease to a multi-factorial syndrome with a complex and varying pathophysiology and prognosis. Before 2004, more than 50 different definitions of AKI (or 'acute renal failure') were in use and the reported incidences, prevalences and outcomes were very variable. The publication of the 'Risk, Injury, Failure, Loss, and End-stage kidney disease' (RIFLE) criteria in 2004, followed by the AKI Network (AKIN) classification in 2007, and the current AKI Kidney Disease Improving Global Outcomes (KDIGO) consensus classification in 2012 [1] have facilitated epidemiological studies showing a high prevalence of AKI worldwide, a link between AKI severity and outcomes, a high risk of short-and long-term complications, including chronic kidney disease (CKD) and major economic consequences [2, 3] (Fig. 1).AKI diagnosis and staging currently rely on a rise in serum creatinine (SCr) and/or decrease in urine output without consideration of the various aetiologies, sites and severity of injury, timing and course of disease and the response to therapy [4]. To compensate for these limitations, descriptors are often added, for instance, 'subclinical AKI' (tubular injury without a SCr rise or with a SCr rise not meeting AKI criteria), 'transient, sustained or persistent AKI' (based on the duration of SCr elevation), 'community' or 'hospital acquired' AKI, and 'recovered AKI' (SCr decrease after peak). However, there are no agreed definitions for these terms [4].