Nanomaterials are finding widespread use in the diagnosis and treatment of diseases. In this talk, I will focus on gold nanocages, a novel class of nanomaterials with hollow interiors and porous walls that can be prepared in relatively large quantities using a remarkably simple procedure based on the galvanic replacement reaction between silver nanocrystals and chloroauric acid in an aqueous solution. By controlling their size and/or wall thickness, the optical scattering and absorption peaks of gold nanocages can be easily and precisely tuned to any wavelength in the near-infrared region (e.g., 700-900 nm, the transparent window of soft tissues) Additionally, their compact sizes, large absorption cross sections (almost five orders of magnitude greater than those of conventional organic dyes), and inertness in biological systems all make them particularly intriguing for biomedical applications. In this talk, I will present some of the most recent advances in the use of gold nanocages for a broad range of theranostic applications, including their use as tracers for tracking by multi-photon luminescence, as contrast agents for photoacoustic and mutimodal imaging, and as radioactive probes for both imaging and therapy. In addition, gold nanocages can selectively target cancerous or diseased tissue, followed by activation as photothermal agents for the selective destruction of the targeted cells. Finally, gold nanocages can serve as drug delivery vehicles for controlled and localized release in response to external stimuli such as NIR radiation or high-intensity focused ultrasound.