This chapter presents the theoretical framework and governing equations for slope stability analysis under rainfall conditions. It begins with a discussion of methods for simulating the heterogeneity of weathered soils, which is critical for accurately capturing spatial variations in slope material properties. The chapter then introduces the numerical implementation of unsaturated seepage analysis, including the treatment of matric suction based on Bishop’s effective stress principle, as well as the constitutive models used to represent the mechanical behavior of the soils. Methods for representing the water-induced weakening effects of soft rock during rainfall are also described, highlighting their importance in simulating realistic responses of rock layers under rainfall conditions. Finally, case studies are presented in FLAC3D, where the shear strength reduction method (SSRM) is employed to calculate safety factors for both two-dimensional (2D) and three-dimensional (3D) slopes under rainfall conditions. These examples illustrate the application of the theoretical framework and constitutive models within numerical simulations. They provide an accurate method of evaluating slope stability and yield valuable insights into the potential failure mechanisms under varying rainfall conditions.