Stomata are epidermal structures that are responsible for modulating the exchange of gases between the plant and the environment. Stomata are formed and patterned by asymmetric cell divisions. The number and orientation of these asymmetric divisions is informed by plant intrinsic signals acting locally (among epidermal cells) or at a distance (from mature to young leaves) and by plant extrinsic factors such as the quantity of light, water and CO(2) in the atmosphere. Recent studies have implicated a set of conserved cell surface receptors and intracellular signaling molecules in the perception and response to developmental cues. Complementary studies have probed the nature of environmental signals and how these signals are transduced from the site of perception to the cells in the stomatal lineage.