Several important characteristic features of the drained and undrained stress-strain properties of saturated soil subjected to monotonic and cyclic loading histories are discussed related to the seismic stability analysis of earth-fill dams. Several basic issues for the proper evaluation of the stress-strain properties at very small strains, strain-non-linearity and shear strength, which is essential for realistic seismic response analysis of soil structure, are discussed. Under drained conditions, compacted soils exhibit significant strain-softening associated with the development of shear bands having a thickness that increases with the particle size, which results in particle size effects on the stability of soil structure. The effects of dry density, inherent anisotropy and different shear mode among different stress-strain test methods, among many important factors, are discussed. It is necessary to properly take into account effects of these complicated influencing factors in practical simplified slope stability analysis in such that the results of adopted simplification procedures employed are well balanced keeping a reasonable conservatism. Loose and dense saturated soils exhibit undrained shear strengths that are significantly lower and higher than the corresponding drained shear strength. With undrained saturated soil, the stress-strain behavior deteriorates and the shear strength decreases by cyclic undrained loading, while the effects of dry density on these properties become more significant by cyclic undrained loading. Simplified practical methods to model these properties are discussed.