The concept of acute kidney syndromes has shifted in recent years from acute renal failure to acute kidney injury (AKI). AKI implies injury or damage but not necessarily dysfunction. The human kidney has an important glomerular function reserve, and dysfunction becomes evident only when more than 50% of the renal mass is compromised. Recent AKI classifications include even slight changes in serum creatinine, acknowledging that this condition is associated with worse outcomes. This, however, still represents a functional criterion for AKI and implies a glomerular filtration rate alteration that may be a late phenomenon in the time course of the syndrome. An early diagnosis of AKI by using tubular damage biomarkers preceding filtration function loss is possible today. Some studies have shown evidence that there is an additional value of new biomarkers not only because they allow a diagnosis to be made earlier but also because they allow a kidney injury to be diagnosed even in the absence of subsequent dysfunction. Only recently, tubular damage without glomerular function loss was demonstrated to be associated with worse renal and overall outcomes. For this condition, the term 'subclinical' AKI has been introduced, challenging the traditional view that a kidney problem is clinically relevant, only when a loss of filtration function becomes apparent. A new domain of AKI diagnosis could then include functional criteria and damage criteria. This may have an impact on the epidemiology, prevention, and management of AKI.