The operation of leaf stomata affects a plant’s photosynthesis, transpiration and respiration, and allows vegetable dry matter to be produced, which is required for production of the tomato fruit. The opening and closing of the stomata can be affected by various factors, and this in turn can have an effect on crop yield. After reviewing the role and functioning of the stomata, this chapter considers the main factors that affect stomata opening and closure, namely, soil water availability, light, CO2, relative air moisture, air temperature and wind. It then considers how these and other factors may interact, reviews the cultivational practices which can promote stomatal opening and briefly cover methods for determining the water status and transpiration rate of plants. The aim of the chapter is to provide novel insights into improving tomato yield by ensuring stomatal pores stay open for longer at appropriate times.