Acute kidney injury (AKI) is a devastating clinical problem that affects a growing number of patients, especially elderly ones, and is associated with high morbidity and mortality. It was previously thought that patients who survive an episode of AKI recover renal function without further sequelae; however, recent population- based studies suggest that this may not be the case. New clinical studies suggest that a strikingly large percentage of patients who have AKI do not fully recover renal function or require permanent renal replacement therapy, and that this population has an important impact on the epidemiology of chronic kidney disease (CKD) and end-stage renal disease. These clinical studies verify animal studies that have established a link between AKI and CKD progression. Future clinical studies are underway to prospectively characterize the natural history of AKI and CKD progression and to identify predictive biomarkers.