The green fluorescent protein (GFP) from the hydromedusa Aequorea victoria and its derivatives have become indispensable imaging devices in cell biology. In previous years, a wide variety of GFP-like proteins were discovered in non-bioluminescent anthozoa. Some of them displayed exciting new properties, including photoactivated changes of the fluorescence emission intensity and wavelength. Photoactivatable proteins offer a high potential as tools for regional optical marking in live cells and tissues. This review aims to give an overview of photoactivatable marker proteins, focusing on the molecular basis of light-induced green to red photoconversion in EosFP.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.