Fluorescent fusion tags have revolutionized many fields in molecular biology. Since the first applications of the Green Fluorescent Protein (GFP) extracted from the jellyfish Aequorea victoria, the photophysical properties of the protein have been improved and modified to optimize their application in various novel in vivo imaging techniques. These applications exploit, unique properties of fluorescence proteins (FPs), such as pH-sensitivity, and Förster resonant energy transfer (FRET), photoactivation, and photoconversion. The engineering of FPs has produced new variants emitting in the blue, cyan and yellow range of the spectrum and created a rich palette of promising new features. In particular the development of species emitting in the far red (RFP) has gained impetus by the discovery of new native FPs from corals (anthozoae). However, most of these newly designed FPs lack important characteristics of the GFP family such as brightness, photostability, and fluorescence at low pH. Therefore, a great need is perceived to better understand the photophysics of the GFP chromophore and its derivatives in the new FPs and how the protein environment controls it.The goals of this project are (1) the validation of experimentally derived models for the photo-induced processes in FPs, (2) the systematic determination of the factors that control absorption/emission wavelength, absorptivity, uorescence quenching via internal conversion. In particular, the effects of electrostatic and steric interactions and hydrogen bonded networks between chromophore and binding pocket on the optical properties and excited-state dynamics will be investigated.