Using the ground as a seasonal thermal energy store is referred to as underground thermal energy storage (UTES). In the vast majority of cases, there are only two basic methods of exchanging thermal energy with the ground: through advection in aquifers using wells and conduction using boreholes. They are referred to as aquifer thermal energy storage and borehole thermal energy storage. While heat pumps (HPs) or chillers are not always used in conjunction with UTES, it is the most common application since most buildings have both heating and cooling loads. In designing HP systems for moderate to large size buildings, it is often the case that the cooling demand is larger than the heating thermal energy demand over the year for large buildings; occasionally it is reversed. In addition, community systems with single-family houses and small residential buildings might have a heating-dominated energy demand. This chapter is largely devoted to the former; however, some notable exceptions are discussed as well.