Since the beginning of the industrial revolution, atmospheric carbon dioxide concentration ([CO₂]) has increased drastically from 270 μmol/mol to around 390 μmol/mol at present, and will exceed 550 μmol/mol by the middle of this century. Rising atmospheric is unanimously the primary driver of global warming, but as the principal substrate for photosynthesis it also directly stimulate the growth and yield of crops. Compared with C₃ crops, our understanding of the response of C₄ crops to future elevated is limited, and most of which are based on studies in closed chambers or open top chambers. Compared with enclosure studies, free air CO₂ enrichment (FACE) experiments are fully open-air trials of crop performance. They provide the most realistic mimic of a future elevated CO₂ atmosphere, and provide perhaps the best opportunity to quantify CO₂ fertilization effects and elucidate the mechanism of observed responses. As the sources of food and forage worldwide, sorghum (Sorghum bicolor) and maize (Zea mays) are the most important C₄ grasses. Following the brief description of the US FACE systems for sorghum and maize, this review paper summarizes the progress of the effects of free air CO₂ enrichment (ambient plus 200 μmol/mol predicted for 2050) on the physiology, growth, yield as well as soil characteristics of the two crop species, and compared the similarities and differences between findings obtained by FACE and enclosure methodologies. Under dry conditions, FACE significantly increased midday photosynthesis of the two crops (up to 23%), however, no CO₂ response detected under wet conditions. Photosynthetic acclimation occurred in leaves of sorghum but not maize under FACE. FACE decreased stomatal conductance (gs) substantially for both wet and dry conditions (up to 35% for mean midday gs), leading to higher leaf temperature and lower leaf transpiration rate, thus resulting in decrease or no change in evapotranspiration, and increase or no change in plant water potential and water use efficiency (WUE). Neither phenology nor plant chemical compositions of the two crops were affected by FACE. FACE increased the growth and yield of two crops to some extend under dry, but not wet conditions. FACE increased mean volumetric soil water content in sorghum but not maize experiment. Soil hyphal lengths of arbuscular mycorrhizal fungi (AMF) and the concentration of one fraction (easily extractable glomalin) of the AMF-produced protein glomalin increased under FACE, resulting in increased water stability of soil aggregates. FACE did not increase N₂O or N-gas emissions (N₂O plus N₂) from an irrigated sorghum production system. For the two C₄ crops, elevated from FACE appears to have reduced gs much more than observed in prior chambers, however, the opposite patterns were observed with the responses of growth and yield. In order to further reduce uncertainties in projections of future global food security, the priority areas for the next generation of C₄ FACE studies should include the interactive mechanisms of CO₂ by genotype, soil moisture, as well as air temperature. Present technological advances suggest that using large-scale FACE facilities to investigate these interactions are now possible.