It is generally accepted that the constraint factor (CF) associated with the indentation of metallic materials by a much harder indentor is in the range of 2.8 to 3.0. Invariably, the CF is assumed to have a constant value in the above range irrespective of the material indented while correlating the hardness of the material indented with its uniaxial strength properties. The objective of the present investigation is to assess the above assumption by evaluating the CF associated with the indentation process as a function of strain in the case of metallic materials exhibiting a wide range with respect to elastic modulus, strength, and strain-hardening rate. The results indicate that the CF is not really a constant but is dependent on the various material properties. The experimental CF-strain relationship observed in all of the test materials has been rationalized on the basis of elastic-plastic and fully plastic indentation models. While the theoretical models can explain the trend of the data, they are not capable of making adequately accurate quantitative predictions. © 1991 The Minerals, Metals and Materials Society, and ASM International.