Plant responses to the projected future levels of CO(2) were first characterized in short-term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO(2) were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO(2) Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO(2) on plants over multiple seasons and, in the case of crops, over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO(2) under completely open-air conditions for nearly two decades. This review describes some of the lessons learned from the long-term investment in these experiments. First, elevated CO(2) stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO(2) improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale. Fourth, elevated CO(2) stimulates dark respiration via a transcriptional reprogramming of metabolism. Fifth, elevated CO(2) does not directly stimulate C(4) photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, the stimulation of yield by elevated CO(2) in crop species is much smaller than expected. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.