This paper is concerned with the design of new and the assessment of existing earth dams. It aims to understand the effect of a soft soil foundation for earth dams. The effects of a compliant foundation are more pronounced under dynamic events such as earthquakes. This study uses dynamic finite element analysis to investigate the dynamic response of earth dams during earthquakes. It is shown that the presence of a soft foundation layer increases the fundamental period of vibration of the dams and, depending on the properties of the dam and foundation, it may lead to amplification or de-amplification of the seismic accelerations. This is an important observation as design codes (e.g. EC8) need to consider the frequency content of the earthquake and the natural frequencies of the dam structure. Therefore this study may lead to a better seismic design of earth dams.