Elastic nonlinearity is responsible for the deviation of a material’s stress-strain response from the linear relationship represented by Hooke’s law. This law can be written in its most general form as σij=Cijklεkl where σij and ɛkl are the stress and strain tensors respectively, and Cijkl is the tensor of the second order elastic constants. The nonlinear elastic behavior can be determined from measurements of the stress dependence of ultrasonic velocities as well as the distortion of ultrasonic waves by the generation of higher harmonics. Consequently, these effects can be used to nondestructively characterize a material’s elastic nonlinearity.