Glomerular filtration rate (GFR) is used for many critical decisions in all patients with kidney disease. In kidney transplant recipients, such decisions may be more frequent and more consequential and time sensitive, including detection of decreased kidney perfusion, obstruction of the urinary tract, consideration of biopsy, selection and dosage of immunosuppressive and antimicrobial agents, and use of contrast imaging. For the general population, current guidelines recommend estimated GFR (eGFR) using serum creatinine (eGFRcr) as the first step in GFR evaluation, with use of eGFR using cystatin C with or without creatinine (eGFRcr-cys or eGFRcys, respectively) or measured GFR (mGFR) as supportive tests.