The wild type green fluorescent protein (GFP) from Aequorea victoria has been extensively investigated with a strong focus on the photochemistry and structural dynamics that are linked with its diverse activities. GFP combines a number of remarkable, and some unique, features that are still intensely researched both experimentally and theoretically. The protein environment effectively inhibits deactivation pathways that are dominant in the isolated chromophore and is therefore responsible for the bright fluorescence. Its p-hydroxybenzylidene-imidazolidinone chromophore acts as a photoacid, and optical excitation triggers ultrafast proton transfer reactions in the active site. The microscopic details of the proton transfer mechanism through a hydrogen bonding network are discussed in this critical review. This property of the wild type GFP has provided the opportunity to characterise the role of the specific protein environment in the proton transfer reactions in comparison to photoacid reactions in the condensed phase. In addition, GFP displays a photochromic side reaction that is uniquely caused by electron transfer from a buried anionic glutamic acid to the optically excited chromophore. This phototransformation property has also been exploited in high resolution fluorescence microscopy techniques. The discussion in this review is extended to include vibrational spectroscopy and structural dynamics (106 references).