Protein intake has been shown to raise glomerular filtration rate (GFR). The difference between peak and baseline GFR following acute protein load indicates renal functional reserve (RFR). In this paper, we review 1) normal glomerular function and its regulating factors, 2) the concept of RFR and the mechanism by which the acute protein load induces hyperfiltration response in healthy subjects and 3) the RFR in patients with renal disease.