Fluorescence imaging is the most powerful technique currently available for continuous observation of dynamic intracellular processes in living cells. However, only a very limited range of biomolecules can be visualized because of the lack of flexible design strategies for fluorescence probes. In our laboratory, it was elucidated that fluorescein which has been widely employed as a core of fluorescence probes could be understood as a directly linked electron donor/fluorophore acceptor system. Fluorescence properties of fluorescein derivatives could be easily anticipated and modulated by controlling the rate of photoinduced electron transfer (PeT) from the donor moiety to the xanthene fluorophore. Further, we found that the opposite direction of PeT from the singlet excited fluorophore to the electron acceptor moiety could be occurred. More than twenty probes for imaging of nitric oxide, beta-galactosidase, highly reactive oxygen species, zinc ion et al. have been developed according to precise and rational design strategies based on PeT mechanism.