This paper illustrates the use of the seismic dilatometer (SDMT) to assess the decay of in-situ stiffness with strain level in different soil types. The approach adopted in this study relies on the ability of the SDMT to provide routinely at each test depth both a small strain stiffness (Go from Vs) and a working strain stiffness (constrained modulus Mdmt derived from the usual DMT interpretation). At various test sites, working strain DMT moduli are compared with reference stiffness decay curves back-figured from (i) the behavior observed under a full-scale test embankment (at Treporti) or footings (in Texas), (ii) from laboratory tests (at L’Aquila, Fucino plain and Po plain) and (iii) various combinations of in-situ and laboratory testing techniques (Western Australia). Typical ranges of the shear strains gamma_dmt associated with working DMT strain DMT moduli are inferred to assist construction of stiffness - strain decay curves for different soil types.