The green fluorescent protein is a key technology in bioimaging. In this critical review, we consider how its various applications can be tailored from knowledge of the excited state chemistry. The photophysics of the basic chromophore in solution are described in detail, and the dominant radiationless decay mechanism is characterised. The quite different photophysics of wild type GFP are described next. The unique excited state proton transfer reaction observed can be used to model proton transfer processes in proteins. Examples where the proton transfer is blocked, or redirected to occur over a low short barrier H-bond are discussed. Finally the photophysics underlying the new generation of photochemically active fluorescent proteins are discussed (155 references).