The use of gold nanoparticles as a platform for light-directed processes in biomedical applications has emerged as a topic of extraordinary promise and potential, with initial successes that have already seen successful clinical translation. By manipulating the geometry of gold nanoparticles, one can control and tune the wavelength at which they absorb light. By tuning the nanoparticle absorption to the near infrared region of the spectrum known as the “water window”, light can penetrate blood and tissue but be strongly absorbed by the nanoparticle. The safe, non-toxic nature of these types of nanoparticles has been well established. This opens the door to a wide variety of in vivo uses, such as contrast agents in near-IR bioimaging, nanoscale light-to-heat transducers for photothermal cancer therapy, and a platform for nanoengineered photothermally triggered molecular and gene delivery strategies. Nanoparticle-based agents can use macrophages as cellular delivery vehicles. Refinements in particle size and properties can enhance both diagnostic and therapeutic efficacy. Nanomatryoshkas, multilayer core-shell nanoparticles, can provide both enhanced fluorescence for imaging and enhanced T1 relaxivities for MRI by, surprisingly, internalizing the fluorophore or T1 contrast agent within their gold shell.  This innovation greatly extends the lifetime of the contrast agent, as well as its clearance time, potentially enabling new cell-focused imaging capabilities.  The ability to enhance T1 MRI contrast can be extended to ions other than the rare earths, providing new non-Gd(III)-based MRI contrast agents that can ameliorate growing health concerns regarding Gd deposition in the brain tissue of healthy patients.  Currently, photothermal cancer therapy is being utilized in human trials for the precise and highly localized ablation of cancerous regions of the prostate, eliminating the highly deleterious side effects characteristic of conventional prostate cancer therapies.