Optical spectroscopy of single plasmonic nanoparticles (NPs) has evolved into a recognized tool for nanoscopic sensing applications, using the sensitivity to the NP's environment,charge, size, shape, and proximity to other NPs. Here, I propose taking advantage of the nanoparticle s minuscule size approaching molecular dimensions in novel ways. Single particle plasmon sensors are in many ways the smallest possible giving unprecedented access to molecular events. The small size amplifies fluctuations by molecular events, allows massive parallel detection of analytes within tiny devices, and to monitor single nanoparticle formation and electrochemical surface reactions in real time.The objective of this project is therefore to develop and explore single-particle plasmon spectroscopy as a novel tool to study such molecular processes. The objective will be reached by (1) building three new setups progressing far beyond current technology and increasing time resolution, spectral sensitivity, and parallelization capability many orders of magnitude, (2) synthesizing nanoparticles with optimal plasmon sensing properties, and (3) simulating plasmon properties to guide the experiments and understand the physics behind the observed phenomena. The single-particle plasmon spectroscopy technique will be applied in four scientific directions to demonstrate its potential: (4) analyzing distance fluctuations of particle pairs linked by (bio-)polymers, (5) recording coverage fluctuations of biomolecules bound to nanoparticles, (6) demonstrating parallel detection of many analytes in multiplexed microfluidic devices, and (7) following particle formation and chemical reactions in a single particle reactor .Single-particle plasmon spectroscopy has the potential to provide a revolutionary new tool to study molecular processes and to become a major commercial analytical tool, especially for pharmaceutical research and development.