The strong optical absorption and scattering of noble metal nanoparticles is due to an effect called localized surface plasmon resonance, which enables the development of novel biomedical applications. The resonant extinction, which can be tuned to the near-infrared, allows the nanoparticles to act as molecular contrast agents in a spectral region where tissue is relatively transparent. The localized heating due to resonant absorption, also tunable into the near-infared, enables new thermal ablation therapies and drug delivery mechanisms. The sensitivity of these resonances to their environment leads to simple affinity sensors for the detection of low-level molecular analytes. Coupled with their general lack of toxicity, these applications suggest that noble metal nanoparticles are a highly promising class of nanomaterials for new biomedical applications.