Abstract: Developments in nonlinear acoustics are followed from gases to liquids to solids. For all materials the nonlinearity parameter can be defined as the negative ratio of the coefficient of the nonlinear term to the coefficient of the linear term in the nonlinear wave equation. With this definition we can compare the nonlinear behaviors of gases, liquids, and solids. However, recent measurements in solids such as rocks and PZT give a much larger nonlinearity parameter. In this case, and similar ones, one must exercise care. Very large nonlinearity parameters often can be explained by inc1uding previously ignored effects in the analysis. As an introduction, the nonlinear behavior of PZT is discussed.