Acute kidney injury (AKI) is associated with worse outcomes after cardiac surgery. It is known that even milder degrees of AKI detected as a small increase in serum creatinine occur frequently and also affect short- and long-term survival. The incidence of AKI after heart surgery varies from 2% to 40% according to different studies. This diversity in the incidence is mainly due to the lack of uniform diagnostic criteria but partly also due to the differences in the examined study cohorts. In the article, we describe three most commonly used definition criteria. Currently, the diagnosis of AKI is based on changes in creatinine values. However, creatinine might increase only 24 hours or even later after the initiation of AKI which suggests that it is inherently unsuitable to serve as a good and reliable early biomarker of AKI. Therefore, new potential biomarkers of kidney injury are described in this review article.