Abstract
Cytokinins represent a class of phytohormones, which are key players not only in many processes important for plant growth and development, but also in the response to changes in their environment. The model for the cytokinin signaling pathway was established at the turn of the last century and many experiments confirmed its validity. In recent years several changes and extensions to the model were necessary to accommodate new findings concerning its components, such as subcellular localization, selective protein degradation and new modes of cross talk. In addition phylogenetic analyses of components of the cytokinin circuitry started to reveal the origin and evolution of the cytokinin regulatory system.