The green fluorescent protein (GFP) from the jellyfish Aequorea victoria can be used as a genetically encoded fluorescence marker due to its autocatalytic formation of the chromophore. In recent years, numerous GFP-like proteins with emission colors ranging from cyan to red were discovered in marine organisms. Their diverse molecular properties enabled novel approaches in live cell imaging but also impose certain limitations on their applicability as markers. In this review, we give an overview of key structural and functional properties of fluorescent proteins that should be considered when selecting a marker protein for a particular application and also discuss challenges that lie ahead in the further optimization of the glowing probes.