Signalling molecules are produced, degraded, modified and transported throughout the development of higher organisms. Understanding their mode of action implies understanding these dynamics in vivo and in real time. Genetically encoded biosensors are being more and more used as tools to 'follow' signalling molecules and their responses inside an organism. This is the case for plants, where important progresses have been made in the development of such biosensors. Here, we summarize the main genetically encoded biosensors built for plant hormones, constructed using diverse components and steps of their signalling pathways.