Stomata are microscopic pores that are critical for plant survival by regulating plant gas exchange and water regulation. They do this by moderating the aperture of the pore through the two flanking guard cells, which change shape in response to large changes in internal turgor pressure. The extent to which guard cells change shape is dependent on the anisotropic growth and deformation of the guard cell wall, and it is this shape change that is fundamental to the opening and closing of the stomata. Modellers of guard cell mechanics and function are increasingly realising that guard cell shape is crucial for understanding how guard cells can function under such conditions, yet there has been little detailed characterisation of guard cell shape change. In this thesis, I present a novel method of imaging Arabidopsis guard cells using confocal microscopy and a way of processing these images to create 3D reconstructions of the guard cells. These reconstructions provide novel quantitative data on guard cell volume, surface area and other geometric parameters when the stomata are both open and closed. These data provide a novel insight into the type of shape changes that guard cells undergo to control pore aperture. I then report on a series of genes implicated in the control of guard cell shape, focussing on a subunit of the ARP2/3 complex, and a family of proteins implicated in regulating cell wall extensibility, the expansins. By a combination of mutant analysis and stomatal functional bioassays, as well as thermal imaging and gas exchange analysis, I provide evidence for a role of the ARP2/3 complex and expansins in guard cell function.