Acute kidney injury (AKI) and chronic kidney diseases (CKD) are clear concepts, based on internationally recognized consensus definitions, that are understood and usefully applied by clinicians and researchers routinely worldwide. Over the last 2 decades, AKI and CKD have not been without controversies, but undeniably they have promoted timely recognition of the importance of deteriorating kidney health and have paved the way for new treatments to slow kidney decline. Central to their functional delineation are the trajectory, timing, and absolute level of serum creatinine—AKI occurs within 1 week, CKD beyond 90 days.