Renal functional reserve (RFR) is the capacity of the kidney to increase its glomerular filtration rate (GFR) in response to physiological or pathological stimuli. The most commonly used stimuli to assess this reserve are an oral load of proteins of animal origin, amino acid infusions, dopamine, glucagon or combinations of them. RFR is calculated as the difference between stimulated and baseline GFR. Vegetarians have lower baseline GFR than the general population and an increased RFR. Subjects with only one kidney and those suffering from chronic nephropathies usually have a reduced or absent RFR despite having normal basal GFR. Quantification of RFR may be useful to detect subclinical renal damage, physiological conditions that reduce baseline GFR, evaluation of potential donors for kidney transplantation, suspected hyperfiltration, detection of renal lability against acute injuries or pregnancy and the evaluation after an acute renal injury when renal function seems to be recovered and residual subclinical damage is suspected.