Fö rster (or fluorescence) resonance energy transfer (FRET) is a nonradiative energy transfer process between two fluorophores located in close proximity to each other. To date, a variety of biosensors based on the principle of FRET have been developed to monitor the activity of kinases, proteases, GTPases or lipid concentration in living cells. In addition, generation of biosensors that can monitor physical stresses such as mechanical power, heat, or electric/magnetic fields is also expected based on recent discoveries on the effects of these stressors on cell behavior. These biosensors can now be stably expressed in cells and mice by transposon technologies. In addition, two-photon excitation microscopy can be used to detect the activities or concentrations of bioactive molecules in vivo. In the future, more sophisticated techniques for image acquisition and quantitative analysis will be needed to obtain more precise FRET signals in spatiotemporal dimensions. Improvement of tissue/organ position fixation methods for mouse imaging is the first step toward effective image acquisition. Progress in the development of fluorescent proteins that can be excited with longer wavelength should be applied to FRET biosensors to obtain deeper structures. The development of computational programs that can separately quantify signals from single cells embedded in complicated three-dimensional environments is also expected. Along with the progress in these methodologies, two-photon excitation intravital FRET microscopy will be a powerful and valuable tool for the comprehensive understanding of biomedical phenomena.Since the discovery of green fluorescent protein (GFP) (1), scientists have been widely employing this gene-encoded fluorescent protein (FP) to investigate the spatiotemporal dynamics of molecules with subcellular resolution. One of the applications of FP engineering is the development of biosensors based on the principle of Förster (or fluorescence) resonance energy transfer (FRET) (2). FRET is a nonradiative energy transfer process between two fluorophores located in close proximity to each other, and its efficiency is strongly dependent on the distance between the fluorophores. More precisely, the energy transfer rate (k t ) from a donor to an acceptor fluorophore is calculated in the following Förster's equation;where J is the overlap of donor emission and acceptor excitation spectra, k 2 is an orientation factor of transition moment (a factor determined by the relative orientation of the two fluorophores), n is a refractive index, R is the distance between the donor and acceptor fluorophores, and k f is the donor fluorescence emission speed. Therefore, FRET efficiency is determined by the distance and orientation of the two fluorophores. To measure FRET efficiency, there are roughly two methods; ratiometric analysis and lifetime analysis. Ratiometric analysis utilizes fluorescence intensity of an acceptor (e.g., yellow FP (YFP)) and a donor (e.g., cyan FP (CFP)) upon the donor excitation. Because accept...