Acute kidney injury (AKI) has emerged as a major public health problem that affects millions of patients worldwide and leads to decreased survival and increased progression of underlying chronic kidney disease (CKD). Recent consensus criteria for definition and classification of AKI have provided more consistent estimates of AKI epidemiology. Patients, in particular those in the ICU, are dying of AKI and not just simply with AKI. Even small changes in serum creatinine concentrations are associated with a substantial increase in the risk of death. AKI is not a single disease but rather a syndrome comprising multiple clinical conditions. Outcomes from AKI depend on the underlying disease, the severity and duration of renal impairment, and the patient's renal baseline condition. The development of AKI is the consequence of complex interactions between the actual insult and subsequent activation of inflammation and coagulation. Contrary to the conventional view, recent experimental and clinical data argue against renal ischemia-reperfusion as a sine qua non condition for the development of AKI. Loss of renal function can occur without histological signs of tubular damage or even necrosis. The detrimental effects of AKI are not limited to classical well-known symptoms such as fluid overload and electrolyte abnormalities. AKI can also lead to problems that are not readily appreciated at the bedside and can extend well beyond the ICU stay, including progression of CKD and impaired innate immunity. Experimental and small observational studies provide evidence that AKI impairs (innate) immunity and is associated with higher infection rates.