This paper provides an overview of the modern understanding of the behavior of soils and foundations during an earthquake based on the papers published in Soils and Foundations over the last fifty years. The most fundamental issues in geotechnical earthquake engineering are the non-linearity of soil under cyclic loading and its implications on the seismic performance of geotechnical structures. The non-linearity of granular materials is essentially anisotropic, whereas the linear or equivalent linear model often used in conventional practices is isotropic. The non-linearity of dry soil is characterized by the difference between the peak and residual strengths, and the increased awareness of the implications of this difference over time has allowed for the development of a generalized methodology for evaluating active earth pressures. The non-linearity of saturated soil under undrained cyclic shear differs fundamentally from that of dry soil, with saturated soil capable of mobilizing 100% of the shear strain in the double amplitude. The cyclic behavior of saturated soil under initial shear is typical of most seismic behaviors of geotechnical structures, including embankments, caisson quay walls, and underground structures. The liquefaction-induced flow failure of geotechnical structures is associated with the steady state of sand in the order of 10 kPa. However, the challenge in the coming decades will be to evaluate the combined effects due to cyclic loading and the steady state. Moreover, the effects of pore water migration are significant in the case of highly permeable geotechnical materials when evaluating settlement of the ground or addressing inter-layered structures of clay and sand. Although current research is elucidating the mechanics of partially saturated sands, the new challenge is to understand combined hazards, such as a combination of earthquake motions and tsunamis. Thus, new approaches and technologies need to be developed.