The Tertiary limestone aquifer of the southeastern United States is a sequence of carbonate rocks that underlies all of Florida, south Georgia, and adjacent parts of Alabama and South Carolina. It is the principal source of municipal, industrial, and agricultural water supply in south Georgia and most of Florida. The aquifer, known as the Floridan aquifer in Florida and the principal artesian aquifer in Georgia, Alabama, and South Carolina, includes various carbonate units of Paleocene to early Miocene age that are hydraulically connected in varying degrees. Very locally, in the Brunswick, Ga., area, a thin sequence of rocks of Late Cretaceous age is part of the system. In general the aquifer consists of either one vertically continuous permeable zone or two major permeable zones separated by a less permeable unit of highly variable water-transmitting characteristics. Aquifer conditions range from unconfined to confined depending upon whether the clayey Miocene and younger rocks that form the upper confining unit have been removed by erosion. Digital model simulation shows that prior to development, most flow in the aquifer occurred in the unconfined and thinly confined areas of northwest and central Florida and southwest Georgia. Springs in these areas are visible evidence of major flow activity. Spring discharge to streams accounted for about 90 percent of the average predevelopment discharge from the regional aquifer. About 18,100 cubic feet per second left the limestone aquifer as spring flow, and 2,500 cubic feet per second discharged as diffuse upward leakage from the confined areas where the vertical head gradient was upward. Most of the 20,600 cubic feet per second recharge necessary to balance total discharge entered the limestone aquifer in the unconfined and thinly confined areas. Because the areas of greatest recharge before development were near the areas of highest discharge, flow paths were generally short. Much water went into and out of the limestone quickly. A very active shallow flow system at the expense of deep circulation has evolved in unconfined and sligptly confined spring areas. Transmissivities commonly exceed 1,000,000 feet squared per day. In contrast, predevelopment flow in the aquifer in the tightly confined areas of southeast and coastal Georgia, far west Florida, and in south Florida was sluggish. In these areas the aquifer is overlain by several hundred feet of sand and clay, except for the outcrop areas along the updip limit of the aquifer. This thick overburden severely retards discharge from the aquifer, causing lethargic flow. Large-discharge springs are nonexistent. The south Florida and southeast Georgia segments of the flow system, which taken together occupy about 50 percent of the regional system, only accounted for slightly more than 3 percent of the predevelopment regional limestone discharge. Transmissivities are on the average lower (generally less than 250,000 feet squared per day) than those in areas of high-flow activity.