Metal nanoparticles have attracted considerable attention due to their interesting properties and potential applications. We have studied the optical and structural properties of different metal nanostructures including aggregates, nanorods, nanoshells, and nannotubes with the goal to optimize their SERS (surface-enhanced Raman scattering) activities. For example, we have very recently demonstrated SERS from single, hollow gold nanostructures. Exceptional sample homogeneity leads to a nearly tenfold increase in signal consistency over standard silver substrates. SERS offers a unique combination of molecular specificity and extremely high sensitivity that few other analytical techniques can offer. SERS based on metal nanoparticles, in conjunction with photoluminescence from semiconductor quantum dots, have been exploited for detection of cancer biomarkers.