The concept and technique of 'SHG-labels' are introduced. SHG-labels are second harmonic-active moieties which can be attached to some molecule or particle of interest that is not SH-active, in order to render the molecule amenable to study at an interface. The labeled molecules may then be studied by surface-selective techniques such as second harmonic generation (SHG) or sum-frequency generation. In the first example of the technique, a protein, cytochrome c, was labeled with sulfhydryl- or amine-specific SH-active dyes. SHG-labels should find use in studying a variety of interfacial reactions.