<p>The unusual plasmonic behavior of atomically thin metallic nanowires is known as quantum confinement effects, and the nanowires severely alter their electronic and optical properties. Such nanowires have localized surface plasmon resonances (LSPRs) that are highly size, shape and environmental sensitive. We examine quantum confinement of LSPRs in the present study through theoretical studies done using the quantum hydrodynamic approach and experimental studies done using electron energy loss spectroscopy (EELS), scanning near-field optical microscopy (SNOM), and ultrafast pump-probe spectroscopy. Nanowires that are a few nanometers in diameter down to a monolayer of atoms were examined. These findings confirm that there are very strong changes in the frequencies of the plasmon resonances, the damping rates, and the broadening of the line widths as the nanowires are made narrower than the Fermi wavelength of the electrons. These observations reveal that classical plasmonic theories fail at the atomic scale and that quantum corrections need to be considered in order to capture accurately the optical response. The presented work offers important insights in the design of plasmonic devices in ultrasensitive sensor applications, in nanoscale-sized waveguides, and quantum information computing systems. The combination of geometry, quantum confinement and plasmonic allows new paths to integrated nanophotonics and next generation quantum plasmonic technologies.</p>