An incipient phase of supernormal glomerular filtration rate (GFR) often heralds diabetic kidney disease (DKD) and many other chronic nephropathies. We review the various definitions set for glomerular hyperfiltration (GH) and the concept of renal functional response induced by haemodynamic and/or metabolic stimuli. The clinical applications and limitations for testing of this response are alluded to. The causes of GH are discussed as perturbations of the physiologic mechanisms that control GFR, emphasizing the unique location of the glomerulus between two sets of resistant arterioles. DKD is discussed as a standard example for the role of GH in initiation and progression of the renal insult. The differences between physiologic and pathologic GH are explored and the morbid consequences of persistent GH are explained. GH is an early pathogenetic alteration in a divergent spectrum of renal disorders whose timely recognition and management might help retard occurrence and progression of chronic kidney diseases