This work presents a comprehensive overview of geothermal system modeling, focusing on the theoretical and practical aspects of simulating geothermal reservoirs and applications. We start by introducing different types of geothermal systems and their unique characteristics. Next, we outline the fundamental governing relationships for geothermal modeling, emphasizing mass, energy conservation, and momentum conservation equations relevant to geothermal energy production. The description then introduces the discretization of these equations, highlighting their role in accurate numerical modeling. Different types of nonlinear formulations, linearization techniques, and linear solution strategies will conclude the description of the numerical solution framework. This chapter also addresses common challenges encountered in geothermal simulation, such as handling single- and two-phase geothermal flow, interactions with nonreservoir lithology, and the main geomechanical signature of geothermal applications. We conclude this chapter with modeling methodologies tailored for various geothermal resources, including those found in fluvial sediments and fractured media, showcasing the diverse range of environments where geothermal energy modeling is applicable. This chapter aims to provide a robust foundation for geothermal resource exploration, development, and optimization through effective simulation practices.