Studies on nonlinear amplification of soil deposits during strong ground shaking are reviewed, because the nonlinearity of soils is one of the most important issues to be investigated for improvement of strong-motion prediction procedures. This issue is recently being discussed by using strong-motion records, although it has been already discussed from the laboratory tests of soil samples. The analyses of the strong-motion records to detect the nonlinearity indicate that the effects of the nonlinearity on the records seem to be different in each case but that the effects become significant when the strain in surface soils is larger than approximately 3×10^-3. Further investigation using stronger records should be necessary for more quantitative evaluation of the nonlinear effects on ground motion.