This paper presents new findings related to the dynamic characteristics of soil-pile interaction in terms of “impedance functions (IFs)”. IFs play an important role to represent the dynamic characteristics of soil-pile systems supporting the superstructure. In general, IFs have frequency dependent characteristics, where the stiffness and the damping change with the excitation frequencies. On the other hand, nonlinearities such as behavior of soil in the vicinity of piles and slippage and separation at the interface between the soil and the pile largely dominates the IFs when a large amplitude of excitation is applied to the pile head. Although both characteristics affect the dynamic response of structural systems, the relationship between the frequency-dependent behavior and the nonlinear behavior of soil-pile systems has seldom been thoroughly investigated. Furthermore, although several lumped parameter models representing the frequency-dependent IFs has been proposed over the past few decades, no model accounting for the nonlinearities of soil-pile systems has yet been developed. To improve the accuracy of the dynamic response estimations in structures, an appropriate model considering both effects is desired. This paper reviews recent experimental results obtained by the author’s team, which demonstrate a convergence trend in frequency-dependent IFs across varying loading amplitudes. Additionally, the author presents an advanced lumped parameter model that incorporates both frequency dependency and nonlinear effects, which is also discussed.