Linking both identity and function within natural microbial communities has long been seen as one of the ultimate goals for microbial ecologists. The phenotypic data obtained from single-cell Raman microspectroscopy can be combined with the phylogenetic resolution of fluorescence in situ hybridization (FISH) to provide a powerful tool for investigating the structure and function of natural microbial communities (Raman-FISH). This chapter provides information about the use of isotopically labeled substrates to track isotope incorporation and hence specific metabolic processes within bacterial functional groups, through the use of Raman-FISH. A general overview of the equipment and techniques necessary for conducting Raman-FISH analysis is followed by a case study highlighting the utility of this technique for discovering the bacterial communities responsible for naphthalene degradation in groundwater microcosms.