Acute kidney injury (AKI) is a serious complication for the critically ill patient. The term has been increasingly adopted over recent years as efforts have been made to capture and better define mild to severe renal dysfunction. Persistent AKI can lead to the subsequent development of renal failure recognized as an important determinant of morbidity and mortality in the critically ill patient. This article explores the clinical implications of AKI for the critically ill patient and how this can have a profound influence on the principal presenting disease and expected outcome.