Ratiometric imaging is widely used to study highly dynamic intracellular ion, voltage or pH changes. The most common application, however, is calcium imaging. Ratiometric imaging is also used for investigating cellular networks, where e.g. relative calcium concentrations are passed among cells or different cell types dynamically change ion, voltage or pH levels. Also FRET assays can considerably benefit from ratiometric imaging, as the signal-to-noise-ratio is greatly improved.