Atmospheric CO(2) concentration ([CO(2)]) is now higher than it was at any time in the past 26 million years and is expected to nearly double during this century. Terrestrial plants with the C(3) photosynthetic pathway respond in the short term to increased [CO(2)] via increased net photosynthesis and decreased transpiration. In the longer term this increase is often offset by downregulation of photosynthetic capacity. But much of what is currently known about plant responses to elevated [CO(2)] comes from enclosure studies, where the responses of plants may be modified by size constraints and the limited life-cycle stages that are examined. Free-Air CO(2) Enrichment (FACE) was developed as a means to grow plants in the field at controlled elevation of CO(2) under fully open-air field conditions. The findings of FACE experiments are quantitatively summarized via meta-analytic statistics and compared to findings from chamber studies. Although trends agree with parallel summaries of enclosure studies, important quantitative differences emerge that have important implications both for predicting the future terrestrial biosphere and understanding how crops may need to be adapted to the changed and changing atmosphere.