Reactive oxygen species (ROS) play a crucial role in a wide variety of processes. Initiation of many different cellular pathways, crosstalk between cells, developmental signalling in planta, programmed cell death and hypersensitive response in connection with plant-pathogen interactions are among the different roles ROS play. On the other hand ROS also cause damage to cellular components at sub-lethal to lethal levels. In photosynthesizing plants the major production of ROS origin from the chloroplast. ROS is a by product from the Photosystem I/II handling of light energy. In nonphotosynthesizing plants the ROS production stems from the mitochondria and peroxisomes as is seen in animal cells. At the Bioimaging Center at KVL we employ different techniques to induce, detect and monitor ROS production, distribution and in and among living plant cells. Both confocal laser scanning microscopy and 2-photon microscopy are used in conjunction with various ROS-sensitive probes to detect and follow the spread of ROS. Techniques where the UV-laser or the 2-photon laser is used to induce ROS synthesis alongside with different ROS-inducing chemical substances are employed at the Bioimage Center. The work presented is based on two-photon laser/UV laser induced ROS development in onion (Allium cepa) epidermis cells in combination with H2O2. We have used the ROS sensitive probe CM-H2DCFDA (Molecular Probes) to visualize the spread of ROS spatially and timely. The ROS signal is monitored over a period of time using two-photon microscopy.