This chapter begins by introducing the fundamental principles of the finite difference method (FDM), followed by a comparison with the finite element method (FEM) in terms of their respective advantages and limitations. It then presents a series of numerical simulations using the commercial software FLAC3D to analyze rainfall infiltration and slope stability in a standard two-dimensional slope model. The discussion also covers the relationship between the permeability of weathered layers and rainfall intensity, and how this interaction affects slope stability under rainfall conditions. The focus then shifts to a comparative analysis of slope stability under rainfall using both two-dimensional (2D) and three-dimensional (3D) approaches. Special attention is given to the influence of slope geometry and groundwater levels on the variability of safety factors across different 2D cross-sections. These variations highlight the limitations of simplified two-dimensional analyses in capturing the complex geometric characteristics of slopes. The findings emphasize the importance of incorporating three-dimensional effects in slope stability assessments, particularly for heterogeneous slopes subjected to complex hydraulic conditions.