Geotechnical models consistently indicate that the stress-strain relationship of soils is nonlinear and hysteretic, especially at shear strains larger than ∼10−5 to 10−4. Nonlinear effects, such as an increase in damping and reduction in shear-wave velocity as excitation strength increases, are commonly recognized in the dynamic loading of soils. On the other hand, these effects are usually ignored in seismological models of ground-motion prediction because of the lack of compelling corroborative evidence from strong-motion observations. The situation is being changed by recently obtained data. Explicit evidence of strong-motion deamplification, accompanied by changes in resonant frequencies, are found in the data from the 1985 Michoacan, Mexico, and the 1989 Loma Prieta, California, earthquakes, the events recorded by the vertical and surface accelerograph arrays in Taiwan, as well as a number of other events throughout the world. Evidence of nonlinear behavior becomes apparent beyond a threshold acceleration of ∼100 to 200 gal. Nonlinearity is considerable in cohesionless soil but may be negligible in stiff soils. The findings of recent years indicate that nonlinear site effects are more common than previously recognized in strong-motion seismology.