Clinical assessment of kidney function is central to the practice of medicine. GFR is widely accepted as the best index of kidney function in health and disease, and accurate values are required for optimal decision making. Estimated GFR based on serum creatinine is now widely reported by clinical laboratories, and in most circumstances, estimated GFR is sufficient for clinical decision making. GFR estimates may be inaccurate in the non-steady state and in people in whom non-GFR determinants differ greatly from those in whom the estimating equation was developed. If GFR estimates are likely inaccurate or if decisions based on inaccurate estimates may have adverse consequences, a measured GFR is an important confirmatory test. Endogenous creatinine clearance is the most common method used to measure GFR in clinical practice but may be difficult to obtain or fraught with error. We review methods for GFR measurement using urinary and plasma clearance of exogenous filtration markers and focus on urinary clearance of iothalamate and plasma clearance of iohexol compared with inulin clearance. We suggest plasma clearance of nonradioactive markers be more widely implemented in clinical settings. Further research is necessary on the impact of the use of measured GFR as a confirmatory test.