This paper summarizes the authors’ studies on heat transfer in ground-coupled heat pump systems. Taking the fluid axial convective heat transfer and thermal “short-circuiting” among U-tube legs into account, a quasi-3-D model has been solved for heat transfer inside boreholes. The transient 2-D temperature response in a semi-infinite medium with a line-source of finite length has also been derived for heat conduction outside boreholes. In order to investigate the impact of groundwater advection on the performance of ground heat exchangers, an analytical solution is obtained for a line heat source in an infinite porous medium with groundwater advection. These explicit expressions have more solid theoretical basis, and can be easily incorporated into computer programs for thermal analysis and engineering design of the ground heat exchangers.