Evaluation of glomerular filtration rate (GFR) is central to the assessment of kidney function in medical practice, research and public health. Measured GFR (mGFR) remains the reference standard, but the past 20 years have seen major advances in estimated GFR (eGFR). Both eGFR and mGFR are associated with error compared with true GFR. eGFR is now recommended by clinical practice guidelines, regulatory agencies and public health agencies for the initial evaluation of GFR, with measured GFR (mGFR) typically considered an important confirmatory test, depending on how accurate the assessment of GFR needs to be for application to the clinical, research or public health setting. Our approach is to use initial and confirmatory tests as needed to develop a final assessment of true GFR. We suggest that GFR evaluation might be improved by more complete implementation of current recommendations and by further research to improve the accuracy of mGFR and eGFR.