Gold nanocages are hollow nanostructures with porous walls that can be simply prepared via the galvanic replacement reaction between silver nanocubes and chloroauric acid. Their optical resonance peaks can be precisely tuned into the near-infrared region, in which the adsorption caused by blood or soft tissue is essentially negligible. Significantly, the strong absorption of gold nanocages makes them attractive as a novel class of contrast enhancement and photothermal agents for cancer detection and treatment. The well-established chemistry for gold also allows them to target specific cells by functionalizing their surface with various moieties such as antibodies, peptides, and DNAs. In this chapter, we focus on their use as a photothermal agent for the ablation of cancer cells and as a contrast agent for the in vivo noninvasive photoacoustic imaging of blood vessels and the sentinel lymph nodes in rats.