Plants are exposed to a wide range of environmental and developmental signals to which they must respond if they are to grow and reproduce. To allow this, plants have evolved complex mechanisms by which these different signals are perceived and transduced to bring about an appropriate physiological response. While these signalling pathways are highly diverse, they all possess two key properties: signal amplification and signal specificity. In addition, many of the components of plant cell signalling pathways are common to all eukaryotes. These include membrane receptors that recognise individual stimuli and numerous proteins, including kinase and phosphatase enzymes, and small molecules that transfer the signals from where they are perceived to their site of action within cells. However, the manner in which these components function in plants can often be different from how they function in other organisms.