Redox processes play a key role in the cells of any organism. These processes imply directed flows of electrons transferred via the so-called redox pairs - compounds present in cells simultaneously in both oxidized and reduced states such as NAD+/NADH, NADP+/NADPH, GSSG/GSH. Up to now, investigation of redox processes in live cells has been hampered by the lack of suitable methods. Genetically encoded fluorescent biosensors represent a new tool for investigation of biological processes including redox ones. Biosensors allow real-time monitoring of messengers, metabolites, and enzyme activities in live systems of different levels of complexity from cultivated cells to transgenic animals. Major types of the known redox-biosensors and examples of their application are presented in this review.