This chapter describes the origins of the study of nicotinamide adenine dinucleotide (NADH) and fluoroprotein to show how they provide a window into the cell function. It is found that mitochondria not only showed a strong fluorescence emission band at 450 nm, but this fluorescence emission band was also linked to the changes of metabolic activity observed in the cytochrome chain with the dual-wavelength spectrophotometer, connecting the NADH pool closely to cytochrome function. It is observed that not only 80% of the signal originated from the mitochondrial aggregate, but the mitochondrial aggregate was also responsive to hypoxia, whereas measurements of the cytoplasm with reference to the extra-cellular space showed little or no change. The exceptionally good signal-to-noise ratio of the NADH fluorescent signal, together with its relatively robust resistance to overillumination, led to exploit flash photography of the myocardium of the perfused heart and to demonstrate not only patchy ischemia in an underperfused heart, but also the spatiotemporal pattern of the distribution of patchy ischemia. The very rapid development of tissuefluorometry, particularly NADH and flavoprotein, has sparked the development of exogenous fluorochromes termed molecular beacons, which seek out particular genetic expressions or label intrinsic substances present in tumors.