The extent to which a crop producer can "manage" the carbon dioxide levels for field crop production either through the addition of manure or crop residues, carbon dioxide fertilization, windbreak, or the application of lime is a much debated, important, and unresolved question. No one solution at any one time may be economical, practical, or effective; but if CO2 levels could effectively be increased, the results might be spectacular and the potential should not be underestimated Considerations of increasing CO2 concentrations must recognize the law of the minimum (see Chapter Three). CO2 enrichment can be effective only if another factor(s) does not limit photosynthesis. Gaastra (1962) stressed this fact, and in addition to the obvious factors such as light, temperature moisture, and fertility, factors that influence water uptake by roots such as the rate of root extension, soil moisture stress, soil aeration, root temperature plus water transport in the upper part of the plant, transpiration rate, absorbed radiation, leaf tissue temperature, wind speed, humidity in the air, etc all of these must be taken into consideration. In addition to all these factors resistance to CO2 movement in the external air near the leaf surface as well as within the leaves themselves may limit CO2 intake Although Wittwer and Robb (1964) mentioned European reports of crops responding to CO2 levels ranging from 20,000 to 30,000 ppm, the possibility of CO2 toxicity cannot be overlooked. Early studies with increased levels of CO2 in greenhouses reported evidence of toxicity possibly due to impurities in the CO2 released. The human safety limit for CO2 is setat 5000 ppm (Anonymous 1960), although much higher levels are safely tolerated for short periods. It is also known that above-normal levels of CO2 do reduce the size of stomatal openings and hence restrict transpiration and CO2 intake. In the presence of decaying manures, however, small amounts of ammonia are released, which have the effect of keeping the stomata open. Because CO2 at high concentrations can close stomata, this has been proposed as a means of increasing water use efficiency (Begg and Turner 1976). The complexities of this subject are great and more research is needed. her than providing a producer with a clear guide to increase crop yields importance CO2 management, this chapter has attempted to show the of CO2 and its complexity in management, as well as to illustrate that crop production is an art as well as a science