This document provides an introduction to geothermal comfort systems. It defines geothermal energy as relating to the internal heat of the earth, noting that the earth acts as a giant solar collector. It distinguishes between high-temperature geothermal from deep underground and low-temperature geothermal, which utilizes shallow earth temperatures via a heat pump. The basic system involves circulating water through underground pipes to transfer heat to and from the ground. During heating the earth serves as a heat source, and during cooling it serves as a heat sink.