The ability to use geothermal heat for any purpose, whether it be power generation, heating a building, or drying fruit, requires some means of transferring the heat from the subsurface to where it needs to be used. In locations where hot springs exist, a natural conduit has developed that provides a means for water to reach the surface from a geothermal reservoir. If the temperature of the water at the spring is sufcient for the application that is being considered, then the only other consideration is whether the ow rate is adequate for the intended application. If there is no surface ow, or higher temperatures are required, and if there is reason to believe a higher temperature resource exists at depth, accessing and using that resource requires understanding of how uid movement occurs in the subsurface, and what the physical attributes are in the subsurface that determine whether a system is a useful geothermal resource. This chapter considers the principles that determine how uids move through the subsurface, the natural constraints that limit water ow, and the basic principles that apply to enhancing water availability.