This paper presents a detailed analysis of the influence of plasticity on the seismic response of pilefoundations. Since soils exhibit nonlinear and irreversible behaviour, it is of major interest to considerplasticity in the design of pile foundations for structures which may be subjected to severe earthquakeloading. The study is carried out using a full three-dimensional finite difference modeling using a realearthquake record. The influence of soil plasticity is investigated on the seismic response of soil-pile-structuresystem for two idealized soil deposits: cohesive and frictional soils. Analyses provide valuable information onthe influence of plasticity on the seismic response of soil-pile-structure systems. They show that the soil statein the vicinity of piles head dominates the piles response and may lead to higher deflection and bendingstresses.