Several methods to assess the stability of slopes during earthquakes were developed during the twentieth century. Pseudostatic analysis was the first method; this involves simply adding a permanent body force representing the earthquake shaking to a static limit equilibrium analysis. Stress-deformation analysis, a later development, involves much more complex modeling of slopes. It uses a mesh in which the internal stresses and strains within elements are computed based on the applied external loads, including gravity and seismic loads. Stress-deformation analysis provides the most realistic model of slope behavior, but is very complex and requires a high density of high-quality soil-property data, as well as an accurate model of soil behavior. In 1965, Newmark developed a method that effectively bridges the gap between these two types of analysis. His sliding-block model is easy to apply and provides a useful index of coseismic slope performance. Subsequent modifications to sliding-block analysis have made it applicable to a wider range of landslide types. It is far easier to apply than stress-deformation analysis and yields much more useful information than pseudostatic analysis.