This chapter discusses Localized Surface Plasmon Resonance (LSPR) spectroscopy is organized as follows: First, the analytical theory of LSPR to explain the physical origin of LSPR. Next a discussion about spectroscopic measurements is performed. Finally, the application of those plasmon resonances is discussed including colorimetric sensing, surface-enhanced Raman scattering (SERS), LSPR-based Rayleigh scattering spectroscopy, and LSPR-controlled fluorescence. The aggregation of metal nanoparticles of appropriate sizes induces inter-particle surface plasmon coupling, resulting in a visible color change. The color change during metal nanoparticle aggregation provides a practical platform for absorption-based colorimetric sensing of variety of target analytes including metal ions, organic molecules, proteins, and nucleic acids, which directly or indirectly trigger the metal nanoparticle aggregation or redispersion. Raman spectroscopy is a vibrational spectroscopy that provides specific information about molecules. Tip-enhanced Raman spectroscopy is another important variation of SERS, which has emerged as a promising technique for in situ chemical analysis on the nanoscale.