In nature the hydraulic system in an aquifer is in balance; the discharge is equal to the recharge and the water table or other piezometric surface is more or less fixed in position. Discharge by wells is a new discharge superimposed on the previous system. Before a new equilibrium can be established water levels must fall throughout the aquifer to an extent sufficient to reduce the natural discharge or increase the recharge by an amount equal to the amount discharged by the well. Until this new equilibrium is established water must be withdrawnfrom storage in the aquifer and conversely the new equilibrium cannot be established until an amount of water is withdrawn from storage by the well sufficient to depress the piezometric surface enough to change the recharge or natural discharge the proper amount. The depression of the piezometric surface is called the cone of depression.The characteristics of the cone of depression in an idealized aquifer of infinite areal extent are considered mathematically. Time is an essential variable in the description of this cone. The time rate of lateral growth of the cone is independent of the rate of discharge by the well and depends only on the physical characteristics of the aquifer. The cone must grow laterally much more rapidly in artesian aquifers than it does in nonartesian. The characteristics of the cone in actual aquifers are then considered. In finely porous artesian aquifers the cone appears to be much like the cone in the ideal aquifer. The cone in non-artesian aquifers must be somewhat warped. The theory out-lined implies that well discharge from extensive water-table aquifers must be a draft on storage in the aquifer for many generations.