Background There are many biological processes that can not be externally monitored with noninvasive imaging modalities such as magnetic resonance imaging (MRI), positron emission tomography (PET) or via x-rays (computerized tomography, CT), because key molecules in these processes are not distinguishable even in the presence of contrast dyes or radioactive tracers. Reporter genes with optical signatures (for example, fluorescence, color or bioluminescence) have been used in cell culture, in small organisms that are relatively transparent (Drosophila) or two-dimensional (plant leaves), and in ex vivo analyses after expression in larger animals. In such assays reporter genes are linked to genetic regulatory elements and can reveal spatial and temporal information about a variety of biological processes at the level of transcription. The goal of real-time analysis of biological events in intact living mammals was recently realized in our laboratories". Two key observations lead to the development of this technology. The first observation was that biological sources of light are sufficiently intense such that the light generated within a mammal can be detected externally". The second observation was that substrate, either endogenous (bacterial) or exogenous (insect), could be made available to biolumi-nescent reporters in living mammals"". We have employed sensitive detection devices to visualize and quantify bioluminescent light by detecting photons that are transmitted through mammalian tissue from internal sources". Several technical advances for imaging weak visible light sources using charged coupled device (CCD) cameras have emerged recently and include microchannel plate intensifiers, Peltier or liquid nitrogen cooling of the detector, and a combination where the intensifier, and not the CCD detector, is cooled. The goal of these technologies is to enhance signal-tonoise ratios by either reducing background (cooled) or increasing signal (intensified). These instruments have broad application in science and industry and are available from a variety of commercial sources. A Hamamatsu intensified CCD (ICCD, model C2400-32) has been used for monitoring infection and gene expression in living mammals in our laboratories.