Acute kidney injury (AKI) is characterized as acute decline of renal function. AKI is frequently combined in hospitalized patients and worsen the outcome of the affected patients. Recently new criteria named RIFLE and AKIN were made to define AKI more uniformly. Recent studies with RIFLE/AKIN showed that even less severe forms of AKI are associated with reduced survival and worse outcome. In this review, we will discuss on the definition, Incidence, Etiology and outcome of AKI.