Reactive oxygen species (ROS) are involved in many signalling pathways and numerous stress responses in plants. Consequently, it is important to be able to identify and localize ROS in vivo to evaluate their roles in signalling. A number of probes that have a high affinity for specific ROS and that are effectively taken up by cells and tissues are commercially available. Applications to intact leaves of singlet oxygen sensor green (SOSG), nitroblue tetrazolium (NBT), di-amino benzidine (DAB) and Amplex Red to detect singlet oxygen, superoxide and hydrogen peroxide are described. Imaging of the probes in the cells and tissues of leaves allows sites of ROS production to be identified.