Acute Renal Failure (ARF) remains a common and significant problem in modern medicine. The epidemiology and causes of ARF vary according to the clinical setting; in intensive care units it is estimated that the incidence of ARF ranges from 10%–25%,1,2 with acute tubular necrosis (ATN) accounting for about 75% of cases.3 Recent data from the BEST kidney investigators has estimated the worldwide prevalence of ARF that is likely to require renal replacement therapy (RRT) amongst ICU patients to be as high as 5.7%, and is associated with a high hospital mortality rate.4 Despite recent major advances in the knowledge of the underlying mechanisms leading to kidney dysfunction, only small improvements have been made with respect to its prevention and treatment, and ARF continues to be a major contributor to in-hospital mortality.5,6 While past clinical trials in patients with ARF have been hampered by the lack of a standardized definition of ARF, recent meetings have resulted in validated stages of ARF as defined by the RIFLE criteria.7,8 These well-defined stages of ARF allow current trials to be standardized with respect to measures of kidney dysfunction, which is essential when evaluating patients for ARF. However, there still remains much improvements to be made in increasing the sensitivity of current markers of acute kidney injury (AKI). Creatinine is currently the most widely-used marker of renal function. Its use in the diagnosis of ARF remains a problem, however, as it often requires as much as a 50% loss in renal function before creatinine levels rise.7 The fact that the many different therapeutic agents that have been tried in ARF have shown very little success is perhaps explained by this delay in diagnosing ARF, which has been compared to beginning the treatment of acute myocardial infarction 48–72 hours after the coronary occlusion.9 For these reasons, there has been a search for better markers of early AKI in recent years, which have seen the advent of several promising new biomarkers of renal function in this setting. This review will discuss some of these promising biomarkers and their potential as useful markers of AKI. A literature search performed on MEDLINE/PubMed using the search terms ‘renal insufficiency, acute’ and ‘biological markers’ yielded 414 results, of which relevant articles were selected from all publication types in the English language, from either human or animal models.10