Most cells acquire iron from plasma transferrin and it is the transferrin receptor that is responsible for the internalization of transferrin-bound iron and its subsequent intracellular release. Plasma concentrations of the receptor reflect cellular receptor numbers and may be determined by enzyme immunoassay on automated analysers. Although determination of receptor concentrations may provide little additional value to existing measures of iron status, particular circumstances may dictate the use of these assays. This review summarizes transferrin receptor physiology and biochemistry, the limitations of the methodology that is currently available and the clinical application of assays (a) in monitoring changes in the rate of erythropoiesis, (b) as an indicator of developing iron deficiency and (c) in identifying iron deficiency in patients with anaemia of chronic disease.