Assemblies of metal nanoparticles, whose conduction electrons interact collectively with light by localized surface plasmon resonance (LSPR), display rich optical properties. Structure-dependent LSPR coupling in these materials produces frequency-tunable absorption and strong electromagnetic fields in gaps between closely spaced nanoparticles ("hot spots"), enabling applications from molecular detection and characterization to energy harvesting and data storage. This sensitivity to nanoparticle spatial organization can render plasmonic assemblies responsive, capable of dynamically reconfiguring due to physicochemical, 15-24 electromagnetic, or mechanical stimuli to form structures with distinct optical near-and far-field spectral features.