Green fluorescent protein from the jellyfish Aequorea victora (GFP) and GFP-like proteins from Anthozoa species encode light-absorbing chromophores intrinsically within their respective protein sequences. Recent studies have made progress in obtaining bright variants of these proteins which develop chromophores quickly and efficiently, as well as novel fluorescent proteins that photoactivate or photoconvert, i.e., become fluorescent or change colors upon illumination at specific wavelengths. Also, monomeric versions of these proteins have been engineered for fusion protein applications. Simple GFP variants and circularly permuted GFP variants have been used to develop fluorescent probes that sense physiological signals such as membrane potential and concentrations of free calcium. Further molecular characterization of the structure and maturation of these proteins is in progress, aimed at providing information for rational design of variants with desired fluorescence properties.