Identifying patients with impaired renal function is crucial in the setting of critical illness. Serum creatinine serves as the gold standard for assessing steady-state renal function, helping to define those with chronic kidney disease (CKD). Although these baseline creatinine values are often not available in the setting of critical illness, CKD, whether defined by serum creatinine or proteinuria, increases the risk of developing acute kidney injury (AKI). Despite delays in elevations following renal insults, serum creatinine remains the standard for assessing acute changes in renal function. Standardized definitions of AKI, using changes in serum creatinine and urine output, have informed the epidemiology of ICU-acquired AKI and have helped define the long-term outcomes in patients who experience AKI. A complex cyclical interplay exists between AKI and CKD, in which CKD predisposes patients to an increased risk of AKI, whereas those with AKI, regardless of baseline renal function, are more likely to suffer from post-AKI CKD. The clarification of the AKI-CKD dynamic remains a work in progress and will be aided by the implementation of novel measures of renal function. Several novel biomarkers of renal function have been proposed to augment serum creatinine in the diagnosis of AKI and CKD. These biomarkers, taken with recent clinical investigations, have laid the groundwork for the impending paradigm shift in risk stratifying and in diagnosing changes in renal function in the ICU.