Acute kidney injury (AKI) occurring in patients admitted to the ICU may result in impaired renal function on long-term follow-up after ICU discharge. The damage induced by subclinical or manifest episodes of AKI may, in fact, produce an irreversible loss of a variable amount of renal mass with deleterious effects on overall renal function. This may be the case even though baseline glomerular filtration rate appears to return to normal but renal reserve is impaired. This may have an important effect on long-term outcomes, including progression to chronic kidney disease. Acute kidney insults should not be considered as isolated episodes but rather a sequence of progressive events that can lead to progressive deterioration of kidney tissue and eventual declines in renal function.