I will describe the use of nanoparticles for the fabrication of label-free, surface optical biosensors by exploiting the localized surface plasmon effect exhibited by individual nanoparticles of gold and silver. We have immobilized gold nanoparticles onto glass slides, functionalized the surface of the gold nanoparticles with biological ligands, and shown that these nanoparticle decorated surfaces enable ligand binding to be detected by the shift in the extinction spectrum of individual nanoparticles. This assay is analogous to conventional, planar SPR with the added advantage of being performed in widely available, low-cost UV-visible spectrophotometers and its facile extension to array based sensors. The extension of this detection modality using gold nanorods and the increased sensitivity afforded by these anisotropic particles will also be discussed. Finally, recent results on reducing the size of the optical transducer to the ultimate limit of a single nanoparticle through the use of dark-field microscopy will be discussed, which opens up the intriguing possibility of obtaining single molecule sensitivity.