Noble-metal nanocages, particularly gold nanocages, are a novel class of nanostructures with hollow interiors and porous walls. These structures are synthesized using a galvanic replacement reaction between metal precursor salts and Ag nanostructures prepared by polyol reduction. The electrochemical potential difference between the two species drives the reaction, with the reduced metal depositing on the surface of the Ag nanostructures. In the case of HAuCl4 as the metal precursor, Au deposits epitaxially on the surface of Ag nanocubes, adopting their cubic structure. Concurrently, the interior Ag is oxidized and removed, leading to the formation of hollow and eventually porous structures known as Au nanocages. This approach is versatile, producing a wide range of morphologies, including nanorings, prism-shaped nanoboxes, nanotubes, and multiple-walled nanoshells or nanotubes, by changing the shape of the initial Ag template. The composition and localized surface plasmon resonance (LSPR) of the metal nanocages can be tuned by varying the amount of metal precursor added to the suspension of Ag nanocubes. These properties make Au nanocages attractive for biomedical and catalytic applications. For example, their large absorption cross-sections and tunable LSPR into the near-infrared region make them suitable for use in optical coherence tomography (OCT) and photoacoustic tomography (PAT) as contrast enhancement agents. Additionally, their photothermal effect, when engineered to have large absorption cross-sections, can be used for cancer therapy through hyperthermia. The galvanic replacement reaction is a general phenomenon that can be exploited to prepare noble-metal nanocages with unique and tunable properties. The potential use of Au nanocages in various biomedical applications, including cancer diagnosis and treatment, is highlighted. Their relative bio-inertness, surface modifiability, and tunable LSPR make them promising nanoscale agents for these applications.Noble-metal nanocages, particularly gold nanocages, are a novel class of nanostructures with hollow interiors and porous walls. These structures are synthesized using a galvanic replacement reaction between metal precursor salts and Ag nanostructures prepared by polyol reduction. The electrochemical potential difference between the two species drives the reaction, with the reduced metal depositing on the surface of the Ag nanostructures. In the case of HAuCl4 as the metal precursor, Au deposits epitaxially on the surface of Ag nanocubes, adopting their cubic structure. Concurrently, the interior Ag is oxidized and removed, leading to the formation of hollow and eventually porous structures known as Au nanocages. This approach is versatile, producing a wide range of morphologies, including nanorings, prism-shaped nanoboxes, nanotubes, and multiple-walled nanoshells or nanotubes, by changing the shape of the initial Ag template. The composition and localized surface plasmon resonance (LSPR) of the metal nanocages can be tuned by varying the amount of metal precursor added to the suspension of Ag nanocubes. These properties make Au nanocages attractive for biomedical and catalytic applications. For example, their large absorption cross-sections and tunable LSPR into the near-infrared region make them suitable for use in optical coherence tomography (OCT) and photoacoustic tomography (PAT) as contrast enhancement agents. Additionally, their photothermal effect, when engineered to have large absorption cross-sections, can be used for cancer therapy through hyperthermia. The galvanic replacement reaction is a general phenomenon that can be exploited to prepare noble-metal nanocages with unique and tunable properties. The potential use of Au nanocages in various biomedical applications, including cancer diagnosis and treatment, is highlighted. Their relative bio-inertness, surface modifiability, and tunable LSPR make them promising nanoscale agents for these applications.