Liquid surfaces decorated with nanoparticles (NPs) afford a promising route to materials with unique and technologically important properties and an ideal platform from which to probe particle adsorption, rearrangement, and structure. Probing these two-dimensional (2D) assemblies can reveal the kinetics, dynamics, and ordering of assemblies that possess inherently interesting optical, chemical, or physical properties. This topic has received renewed interest from the condensed matter community, yet directly visualizing the assemblies in-situ and leveraging the unique properties of NPs at these interfaces remains challenging. The thesis work detailed herein connects 2D NP assembly fundamentals to striking plasmonic, photonic, electronic, and reconfiguration properties afforded by the inherent NP properties and the fluid nature of the interface. The real time NP structure and dynamics at an ionic liquid–vacuum interface were revealed by single particle tracking using scanning electron microscopy (SEM). Upon approach to jamming, the dynamics trends were strongly particle size dependent due to the increase in particle lubrication facilitated by the ligand brush layer. Unfortunately extending the in-situ SEM technique to liquid–liquid interfaces was not possible; instead, a suite of complementary techniques including: GISAXS, GIXPCS, UV-Vis reflection spectroscopy, and pendant drop tensiometry, were used to probe the assemblies. The phase separation of plasmonic and non-plasmonic NP mixtures was examined and showed that NP size disparity and non-plasmonic NP number fraction significantly influence the adsorption process and equilibrium packing of gold NPs. Electronic or photonic properties were incorporated into liquid–liquid interfaces by the careful selection of the polymer used. Phytic acid (PA) and sulfonated polyaniline (S-PANI) ink phases showed enhanced electrical performance due to the dense packing of PA/S-PANI complexes at the interface. 3D printed all-liquid circuits could self-repair on demand after the conductive pathway was mechanically broken. Block copolymers were used to generate well-ordered interfacial films, where all colors across the visible spectrum were obtained. Finally, future directions and the design of a liquid cell TEM are discussed. Understanding how collective NP characteristics direct interfacial self-assembly is a promising step in improving current nanotechnologies to confront challenges in renewable and green energy, energy storage, and materials transportation.