The galvanic replacement reaction between a Ag template and HAuCl4 in an aqueous solution transforms 30–200 nm Ag nanocubes into Au nanoboxes and nanocages (nanoboxes with porous walls). By controlling the molar ratio of Ag to HAuCl4, the extinction peak of resultant structures can be continuously tuned from the blue (400 nm) to the near-infrared (1200 nm) region of the electromagnetic spectrum. These hollow Au nanostructures are characterized by extraordinarily large cross-sections for both absorption and scattering. Optical coherence tomography measurements indicate that the 36 nm nanocage has a scattering cross-section of ∼ 0.8 × 10–15 m2 and an absorption cross-section of ∼ 7.3 × 10–15 m2. The absorption cross-section is more than five orders of magnitude larger than those of conventional organic dyes. Exposure of Au nanocages to a camera flash resulted in the melting and conversion of Au nanocages into spherical particles due to photothermal heating. Discrete-dipole-approximation calculations suggest that the magnitudes of both scattering and absorption cross-sections of Au nanocages can be tailored by controlling their dimensions, as well as the thickness and porosity of their walls. This novel class of hollow nanostructures is expected to find use as both a contrast agent for optical imaging in early stage tumor detection and as a therapeutic agent for photothermal cancer treatment.