This article discusses selected issues addressed by a 2002 field torsional cylindrical impulse shear testing program carried out at the National Geotechnical Experimentation Site on Treasure Island in the San Francisco Bay. The impulse shear test is a new in situ geotechnical testing method that provides detailed information, for soil deposits, on in situ nonlinear inelastic shearing deformation characteristics needed for dynamic geotechnical earthquake analysis procedures. One purpose of the Treasure Island testing program was to initiate a definitive verification of this capability. Previous field testing programs we had carried out were preliminary evaluations that focused largely on operability issues. Additionally, recent findings suggest that the impulse shear test may have the potential for providing reasonably precise indications of in situ resistances of soil deposits to liquefaction and related deformations. A second purpose of the Treasure Island testing program was to further explore this possibility. The Treasure Island site is a particularly relevant site for the subject investigation. The site consists of a layer of saturated loose sandy soils underlain by a medium-stiff clay layer. Such a sequence has proven to be extraordinarily hazardous during past earthquakes. The clay layer can amplify ground motions greatly, and the sandy layer tends to liquefy. The results of our testing program suggest that the impulse shear test is (1) a viable means for estimating in situ nonlinear inelastic shearing deformation characteristics of soil deposits and (2) a promising approach for obtaining reasonably precise indications of in situ resistances of soils to liquefaction and related deformations.