Recently, nanoparticles have become the platform for many sensing schemes. In particular, the utilization of the optical response of nanoparticles to changes in their nanoenvironment has served as a signal transduction mechanism for these sensing events. For example, silver nanoparticle arrays synthesized using nanosphere lithography have served as an ultrasensitive detection platform for small molecules, proteins, and antibodies with the detection limit of 60,000 and less than 25 molecules/nanoparticle for hexadecanethiol and antibodies, respectively. While this approach is low cost and highly portable, one limitation of the array platform is that the signal arises from approximately 1x106 nanoparticles. A method to improve the overall number of molecules detected would be to decrease the number of nanoparticles probed. Recently, single nanoparticle sensing has been accomplished using dark-field microscopy. A 40 nm shift in the localized surface plasmon resonance induced from less than 60,000 small-molecule adsorbates has been monitored from a single Ag nanoparticle. Additionally, streptavidin sensing has also been demonstrated using a single Ag nanoparticle. Detection platforms based on nanoparticle arrays and single nanoparticles will be discussed and compared.