Heat pump technology is one of the most sophisticated engineering accomplishments to come out of the twentieth century. Heat pumps are simple devices that operate at the highest efciency levels accomplished by heat-transporting systems. They are, in essence, close analogs to actual Carnot cycles, operating on precisely the same principles as that which Carnot imagined in his thought experiment. They transfer heat in ways that can accomplish both heating and cooling, while consuming a small fraction of the amount of energy they move. Their great advantage comes from the fact that they move heat that already exists, using basic thermodynamic principles and thus do not require that heat be generated. Because generating heat (through combustion, friction, or some other means) is usually inherently less efcient than moving preexisting heat, a heat-transfer engine provides efciencies that are difcult to match through other means. In this sense, they provide the ideal method to satisfy energy demands for heating and cooling buildings and spaces. This chapter considers the principles that need to be addressed when coupling heat pumps to the earth’s nearsurface heat reservoir, design concepts for such systems, and issues that need to be considered to successfully implement such an application.