Protein kinases are key modulators of intracellular signal transduction cascades, which determine various events in neuronal cells such as replication and differentiation. For many years, protein kinases were �analyzed mostly by biochemical methods, which could handle the cells only en masse. For a better understanding of the role of kinases in neuronal cells, one would like to know the subcellular distribution of kinase activities and to follow a particular kinase activity for a specific period in a single cell. Genetically encoded biosensors based on the principle of F�rster (or fluorescence) resonance energy transfer (FRET) and fluorescent proteins have been developed to accommodate such requirements. The method involves expression of the FRET biosensors in neuronal cells, time-lapse imaging under fluorescence microscopes, image processing, and quantification of FRET. This technique could be applicable to living organisms ranging from Caenorhabditis elegans to mouse, permitting visualization of spatio-temporal regulation of kinase activities and systemic understanding of the signaling networks in living animals.