Metal or doped metal oxide nanoparticles can be integrated into soft materials to harness light for sustainable energy, biomedical, and sensing applications. Optical responses of such plasmonic materials are sensitive to the spatial organization of their nanoparticles due to coupling of localized surface plasmon resonances, making controlled assembly a promising platform for plasmonic superstructures. To guide development of such materials, it would be useful to discover a small set of experimentally tunable parameters that govern their optical properties. For example, theoretical, computational, and experimental studies have established "plasmon rulers" for nanoparticle dimers and superlattices which correlate structure-induced changes in their resonant frequency to the distance between nanoparticle nearest neighbors. Optical properties of small clusters are also understood in terms of their structure based on extensive experimental characterization and modeling. Mean-field and effective medium approximations, e.g. Maxwell Garnett or Bruggeman theories, ignore structural details, but offer simple approximate predictions for the effective permittivity of single-or multi-component nanoparticle mixtures in terms of only the volume fractions of their components. Quantifying structure-optical property relations for extended disordered assemblies, where structural metrics beyond the volume fraction and the nearest-neighbor distance have important implications, remains challenging. Examples include concentrated colloidal dispersions, films, and glasses, large disordered clusters or aggregates, colloidal gels, and disordered metasurfaces or multilayers. Like superlattices, disordered assemblies can exhibit collective plasmon resonances reflecting long-range, many-body interactions, though the connection of optical properties to structure is poorly understood. Despite advances in electron microscopy and X-ray scattering for studying ordered plasmonic assemblies, in situ experimental probes alone cannot fully characterize structure across the hierarchy of relevant length scales in disordered systems. Electron microscopy can provide high-resolution information about the colloidal building blocks but is not well suited to gather ensemble-level information about three-dimensional assemblies, and subwavelength nanoparticles are too small for their coordinates to be spatially resolved using confocal microscopy.