This paper reports a set of high time-resolution analytical models for heat transfer of GHEs, including a full scale line-source model for heat transfer outside U-shaped pipes and a quasi-3D heat transfer model for the circulating fluid inside U-shaped pipes. The full scale line-source model is a composite expression consisting of a composite medium line-source solution for short-term temperature responses, an infinite line-source solution for mid-term temperature responses, a finite line-source solution for long-term temperature responses, and a finite line-source solution for the thermal interaction between adjacent boreholes. The quasi-3D model for heat transfer inside U-pipes tackles the variation of the fluid temperature along the U-shaped channels. The fluid temperatures in the descending and ascending legs are derived as functions of time and borehole depth. The quasi-3D model, together with the full scale G-function, constitutes a complete theoretical model for heat transfer by ground heat exchangers, providing a computational-efficient approach to computer simulation, analysis, and optimum design of ground-coupled heat pump and ground heat storage systems.