Acute kidney injury (AKI) is a common disease in the acutely ill patient population, as a singular diagnosis or a complication of sepsis, causing significant mortality and morbidity. Progress in diagnosis, treatment, and research in AKI has been limited by the lack of a universally accepted clinical definition. The clinical definition of AKI onset and progression, early diagnostic indicators, and understanding the unique pathophysiology of AKI are requisite to early treatment and management and ultimately positive patient outcomes. This article reviews the advances in defining and staging AKI on the basis of international consensus statements. An update on the most recent concepts affecting renal pathophysiology in AKI is also presented. Current clinical tools used in diagnosing and monitoring AKI, including the development of renal biomarkers, are discussed.