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The regulation of gas exchange at the leaf level is a key factor for plant survival under a fluctuating environment (Buckley, 2005). In this context, control of stomatal opening and closure is the evolutionary solution to balance water loss with CO uptake and yield. A decrease in leaf/root water potential resulting from soil drought is typically accompanied by an elevated level of abscisic acid (ABA), which is well established as a stress hormone (Davies et al., 2005). ABA is a central component in drought-stress sensing leading to efficient stomatal control, thereby avoiding deleterious yield losses during stress conditions. Depending on the crop species, or its growing environment, different strategies for yield-optimization need to be chosen (Araus et al., 2002; Chaves and Oliveira, 2004). ABA effects are modulated by the levels of and sensitivity to other hormones, in an interdependent network. Unraveling the complex regulatory mechanisms of stomatal control between hormones, second messengers, ion channels and other classes of implicated proteins will lead to new insights in how to tailor plants to take maximum advantage of the available natural resources (Li et al., 2006). Possible strategies are either to trigger an earlier stress response without a negative impact on yield, or to attenuate the plant stress response so that assimilation will increase. These desired traits can be brought about by overexpressing or downregulating the expression of specific genes involved in the complex and possibly redundant signaling network of stomatal responses. This chapter provides an overview of the mechanisms behind the changes in stomatal movements under water-limiting conditions, including hormonal regulation and developmental influences