Over the past several years, an increasing number of fluorescent probes — from both natural and laboratory sources — have made possible noninvasive imaging of live cells and even whole organisms. And, according to Roger Y. Tsien and colleagues at the University of California, San Diego, in La Jolla, fluorescence techniques will remain a key part of biological research for years to come. In a review of the fluorescent tools available to researchers, the authors note that fluorescent probes — such as GFP and other proteins derived from Aequorea victoria or man-made inorganic crystals, such as quantum dots — provide high spatial and temporal resolution in most imaging applications. Furthermore, the power of fluorescence imaging continues to expand, with improvements not only in the probes, but also in the instruments, data analysis tools and strategies used to implement them. Photo courtesy of National Center for Microscopy and Imaging Research/Ben Giepmans. The scientists discuss recent advancements in fluorophores, including small organic dyes, fluorescent proteins and quantum dots; immunolabeling and genetic tagging as a means of attaching fluorophores to proteins of interest; and methods with which to study protein expression and localization in primary cells and in fixed tissue samples. They also describe techniques using fluorescent probes to observe protein synthesis and turnover, diffusion, conformational changes and interactions with other proteins in live cells, as well as ways in which to improve spatial resolution past the diffraction limit and depth of penetration into tissue. The authors report that further progress in the development and use of small organic dyes is likely to be incremental, but that there is no apparent limit to the variety of reporter molecules incorporating fluorescent proteins to monitor cell biology. (Science, April 14, 2006, pp. 217-224.)