In this chapter, we present a few applications where numerical modeling can be applied to solve practical problems occurring in the exploration and exploitation of geothermal resources. First, we give in section 9.1 a short and very simplified introduction to geothermal energy, its applications for direct uses and for power generation and its benefits compared with other renewable energy resources and with fossil fuels. In that section we also give a simplified description of geothermal reservoirs. The descriptions presented here are very fundamental and are only aimed to introduce those readers, who are not familiar with geothermal energy or geology to these topics and to indicate them those references where they can find more detailed information.