This book discusses ideas associated with development of groundwater at the macro scale, including how well pumpage affects surface water, groundwater storage, and the long-term sustainability of groundwater development. We do not focus on well hydraulics, well interference, or design of wells or well fields. Instead, we begin with where water comes from when one pumps a single well and proceed to the scale at which an entire aquifer system is developed. These ideas date back to a paper by Theis (1940) on the source of water derived from wells. Theis (1940) noted that all pumpage is balanced by a loss of water somewhere, with the loss during early time coming largely from storage. As time advances, pumpage tends to be balanced increasingly by some combination of increases in recharge and decreases in discharge.