One of the key variables in evaluating the response of geotechnical materials during strong earthquake shaking is their nonlinear behavior. This behavior is often expressed in terms of the nonlinear reduction in shear modulus with increasing shearing strain and the corresponding increase in material damping ratio. Currently, there is total dependency on laboratory testing to measure these properties. A generalized test method is under development at the University of Texas at Austin to measure nonlinear soil properties in situ. The method involves applying static and dynamic loads at the surface of a soil deposit using a mobile, electro-hydraulic shaker and measuring the dynamic response of the soil mass beneath the loaded area using embedded instrumentation. The resulting field test is a load-controlled dynamic test that induces nonlinearity in the soil. Initial prototype studies have focused on measuring linear and nonlinear shear moduli over a range in stress states. These studies are discussed, and measurments in a sand deposit are presented. The linearity and nonlinearity measured in situ compare well with laboratory results and empirical trends.