Gold nanoparticles have unique optical and chemical properties that make them ideally suited for a number of applications in bionanotechnology, including optical probes, targeted drug delivery, and programmed materials synthesis. The recurring theme is the versatility of gold nanoparticles and how this versatility can be exploited to adapt gold nanoparticles to fit into different chemical environments—from aqueous suspensions to the hostile conditions of a cell culture or the human bloodstream. Herein, the optical and chemical properties of gold nanoparticles are described, and a number of specific applications within bionanotechnology are reviewed.