Cytokinins are essential hormones for the proper growth and development of plants. They exert their actions through the phosphorylation of two-component signaling factors. The two-component elements in cytokinin signaling display not only overlapping, but also specific functions throughout a life cycle. These elements regulate the development of shoots, roots, and inflorescence meristems in Arabidopsis; shoot meristems in rice; and nodule formation in the lotus. They are also involved in interactions between plants and pathogens. In this review, we examine the mechanism for signaling events initiated by cytokinins in Arabidopsis.