Acute kidney insufficiency (AKI), or injury, is common in the critically ill patient. Minimal increases in serum creatinine (Scr) have been associated with greater morbidity, mortality, and hospital cost. In 2002, the Acute Dialysis Quality Initiative (ADQI) proposed a consensus definition (the RIFLE classification) which was modified after continuing evidence suggested that small changes in Scr (≥0.3 mg/dL) led to worsening outcomes. This group, known as the Acute Kidney Injury Network (AKIN), suggests 3 stages of worsening kidney function. Such definitions may aid in identifying patients at greatest risk and further the development of preventive strategies. This review will focus on the epidemiology and etiology of AKI as well as provide a mechanistic description of drug-induced AKI. In addition, a brief review of continuous renal replacement therapies is provided.