A pair of guard cells forms a stomatal pore in the epidermis of plant aerial tissues. Stomatal pores can open and close to regulate gas exchange in response to environmental stimuli, such as the plant hormone abscisic acid (ABA), water status, temperature, and light conditions, CO₂ exchange between plants and the atmosphere, and bacterial invasion. For example, elevated ABA and CO₂ levels induce stomatal closure, whereas blue light causes stomatal opening. It is hypothesized that guard cells possess signaling mechanisms that integrate multiple environmental signals to persuade stomatal movement and to regulate plant survival under various conditions. Recent integrated approach has allowed us to gain a glimpse of the network of signal transduction mechanisms in the modulation of stomatal movement. Molecular genetic studies together with cell biological analysis have revealed many signaling components and pathways involved in the regulation of stomatal movement. Genomics- and systems-based approaches have led to the generation of models of the signaling network in guard cells. This chapter focuses on recent advances in our understanding of the mechanisms of guard cell signaling and the interaction of multiple signals to modulate guard cell movement.