Cytokinins are phytohormones involved in the regulation of many physiological and developmental processes in plants. Generally, they stimulate cell division and help to control root and shoot meristem activity, but they further exert control on processes related to leaf senescence, nutrient uptake and pathogen resistance. In flowering plants, the cytokinin system has evolved to a high degree of complexity and the key elements of cytokinin biosynthesis, activation, inactivation, degradation and perception are well understood on a molecular level. Cytokinin activity is interconnected with many other signalling pathways including those of other hormones. Cytokinin signalling already appears at the evolutionary level of early divergent land plants. There is evidence that in future crop plant properties and productivity improvements could be achieved by making use of the increasing knowledge of the cytokinin regulatory network.