Nanoparticles with unique optical properties, facile surface chemistry, and appropriate size scale are generating much enthusiasm in molecular biology and medicine. Noble metal, especially Au, nanoparticles have immense potential for cancer diagnosis and therapy on account of their surface plasmon resonance (SPR) enhanced light scattering and absorption. Conjugation of Au nanoparticles to ligands specifically targeted to biomarkers on cancer cells allows molecular-specific imaging and detection of cancer. Additionally, Au nanoparticles efficiently convert the strongly absorbed light into localized heat, which can be exploited for the selective laser photothermal therapy of cancer. We discuss recent advances in the study and use of selectively targeted Au nanospheres in cancer photodiagnostics and photothermal therapy. By changing the shape or composition of Au nanoparticles, the SPR can be tuned to the near-infrared region, allowing in vivo imaging and photothermal therapy of cancer. The use of Au nanorods and silica-Au core-shell nanoparticles for in vivo cancer detection and therapy is discussed.