Hardness is a concept much easier to use than to define. It is perhaps best defined by the means which it is determined. Hardness is measured in two main ways, both of them empirical methods.

The first, and oldest, method is that of scratch testing where if material A can scratch material B, then material is harder than . The Mohs hardness scale is a system of reference materials against whose hardnesses a sample is compared.

The other method is a so-called "indent test," in which an "indenter" of defined geometry is pressed into the surface with a well-defined force. The penetration of the indenter is then related to a hardness number. These scales allow a more quantitative value than scratch tests. Popular hardness scales include the Rockwell hardness scales, Brinell hardness scale, and the Vickers hardness scale.