The paper summarizes the results of a series of in situ and laboratory tests aimed at investigating the small strain shear modulus G0 of six different Italian natural clays. The obtained experimental data were analyzed to assess the influence on G0 of the following factors: - soil state intended as a combination of the current void ratio e and the acting effective stress; - overconsolidation due to unloading and to aging; - the plane of polarization with respect to the principal stress directions for seismic body waves. All laboratory tests have been performed on specimens of high quality undisturbed samples that were reconsolidated to the best estimate of the in situ geostatic effective stresses. For the covering abstract, see IRRD 892179.