Acute kidney injury (AKI) is recognized as an independent risk factor for morbidity and mortality. Unfortunately, this syndrome was historically underdiagnosed due to inconsistent definition of AKI as well as insensitive and nonspecific diagnostic tools. Recent advances in defining AKI, understanding its pathophysiology, and improving its diagnostic accuracy have an impact in disease management and clinical outcome. Prompt recognition and treatment of AKI still remains the cornerstone of clinical management of this syndrome. This chapter focuses on the recent advances in diagnosis of AKI using novel serum and urine biomarkers. The role of neutrophil gelatinase-associated lipocalin (NGAL) in pathophysiology and diagnosis of AKI is presented. A detailed analysis of the biology of NGAL and presentation of laboratory methods of measurement is also provided. The role of NGAL as biomarker beyond the boundaries of nephrology is also presented.