A study group of the New Zealand National Society for Earthquake Engineering has recently completed recommendations for the seismic design of storage tanks in a form suitable for use as a code. A knowledge of site response is an integral part of seismic analysis, unfortunately providing guidelines on assigning relevant soil parameters (shear modulus and damping in particular) cannot easily be resolved in a code format. However, as shear modulus (Gs) is referred to directly in the recommendations, it was decided to provide this technical note to enable some guidelines for its assessment to be given. It is an involved problem which requires a great deal of judgment on the designer's behalf if a realistic value of Gs is to be attained. Most available data on Gs has been developed for either sands or saturated clays although there has been a limited amount of work done on gravelly soils. Because most soils have curvilinear stress-strain relationships, it will be appreciated that the shear modulus is not constant but is usually expressed as the secant modulus determined for a specific value of shear strain.