Fluorescent molecules – i.e. substances which can be stimulated to emit light – are extremely valuable tools in biological research and medical diagnosis. Fluorescence can be used for instance to analyze the regulation and expression of genes, to locate proteins in cells and tissues, to follow metabolic pathways and to study the location and migration of cells. Of particular importance is the combination of fluorescence imaging with novel techniques that allow tomographic three-dimensional visualization of objects in living organisms. At the Helmholtz Zentrum München – German Research Center for Environmental Health together with the Technische Universität München an own institute is concerned with the development and refinement of such new technologies: the Institute for Biological and Medical Imaging headed by Professor Vasilis Ntziachristos.