The impact of seismic loading on municipal solid waste landfills is investigated numerically in the current study. Initially, the role of the main parameters involved in the linear or nonlinear dynamic response of landfills is determined in terms of developed inertial acceleration. The most important factors are the geometry, the waste material properties, and the characteristics of the seismic excitation. Concerning the evaluation of the potential failure mechanisms, the effect of the main parameters on the slope stability is underlined, and a comparison between circular slope instability and base sliding is presented. Furthermore, according to the failure envelopes adopted in the literature for the shear strength of the waste material, a decoupled procedure is utilized to estimate the developed landfill deformations. Finally, the generation of parasitic vertical acceleration (due to the multidimensional nature of the problem) is taken into account in stability assessment, while its effect on the resulting deformation is determined.