Histochemistry, which enables us to visualize the precise distribution of target molecules within living organisms, is an essential strategy in life science research. Of various histochemical modalities, confocal microscopy provides not only the existence of target molecules, but also their spatiotemporal dynamics. Because of the high spatial resolution of the confocal images, fluorescence intensities for specific molecules reflect their concentrations: the higher the intensities, the more abundant the molecules present. In addition, rapid scanning confocal imaging provides precise concentration changes in functional molecules, reflecting cellular functions. Here I would like to exemplify fluorescent imaging of intracellular Ca²⁺ dynamics, cellular membrane potentials, and mitochondrial states of the cardiomyocytes in the heart. Photodynamic modulation of cellular functions will also be demonstrated.