This thesis present analysis methods for describing the seismic ground response and the influence of soil overlaying a firm elastic bedrock. The site is not for a specific location but the analyses are performed for a soil profile with soil to a significant depth and for an area with large seismic hazard. The thesis describes the altering of the earthquake ground motion as it propagates through the soil from bedrock to surface. The ground motion analyses are compared with the approach described by Eurocode 8,EN1998. Detailed analyses are performed by an equivalent linear and by a nonlinear modelling of the soil. The equivalent linear model includes strain dependant dynamic soil properties and rate independent damping. The nonlinear model is performed by modelling the soil with the extended Masing model which furthermore includes the hysteretic behaviour and plastic deformation of the soil. The comparison of the analyses methods indicates that the nonlinear model gives the best results. The equivalent linear model also gives reasonable results for well described input motions and for lower strain levels of the soil. For large strains the equivalent linear model fails to describe the nonlinear behaviour of the soil. The procedure given by Eurocode EC8 gives satisfactory results for low period input motions but fails to describe the amplification of the motion at higher period. For bedrock input motions with significant energy at high periods this amplification results in an underestimation of the surface motion and in such cases the equivalent linear and the nonlinear models are recommended.