Fluorescence correlation spectroscopy (FCS) is a powerful tool for detecting molecular dynamics through analyzing the intensity fluctuation emitted by biomolecules diffusing in and out of a focused light [1 – 3]., biomedicine, biophysics, and chemistry. Its theoretical underpinning originated from L. In principle, light is focused in an area of the sample and the fluctuations in the fluorescence intensity in this. In Chapter 1 we briefly introduce absorption and fluorescence.