Surface plasmons are collective and coherent oscillations of conduction band electrons in metal nanostructure which enable coupling of photons to electrons at a metal dielectric interface. Plasmonic nanostructures have gained much attention due to their ability to confine, tune and manipulate light for specific applications simply by varying their geometries and local dielectric environment. This thesis will focus on designing and studying fundamental plasmonic properties of Au nanostructures for applications in photothermal cancer therapy, chemical sensing, optical waveguiding, and room temperature gas phase photocatalysis. First, this thesis focuses on spherically concentric nanoparticles, a rudimentary “nanomatryushka”, composed of a silica-coated gold nanosphere surrounded by a gold shell layer. These nanoparticles were synthesized using wet chemistry technique and were found to possess exceptional geometrically tunable optical resonances in a compact, sub-100 nm size. Changing the internal geometry of the nanoparticle not only shifts its resonance frequencies, but can also strongly modifies the relative magnitudes of the absorption and scattering cross sections, independent of nanoparticle size. In addition the inherent asymmetry of each individual Au/SiO2/Au nanomatryushka generate multiple Fano resonances due to the overlapping bright superradiant and dark subradiant plasmon modes. Fano resonances have immense potential for single particle localized surface plasmon sensing applications. Next, this thesis investigates a new class of waveguiding consisting of chains of fused heptamer nanodiscs. This novel waveguiding structure transports electromagnetic energy via magnetic plasmon resonance mode. In this new geometry, heptamer structure serves as a benzene-like subdiffraction limit building blocks which support antiphase magnetic plasmons with “antiferromagnetic” behavior in multiple repeated structures. By repeating the heptamer units, this waveguide enables low-loss magnetic plasmon propagation along linear chains, steering over large-angle bends and splitting. It has numerous potential uses in energy transport, data storage, near-field microscopy, and other nanophotonic applications. Finally, this thesis explores the use of Au-photocatalysts as multifunctional catalysts for enhanced reactivity and efficiency. Au-photocatalysts were used for room temperature dissociation of H2 on Au nanoparticle surface using visible light. Surface plasmons excited in the Au nanoparticle decay into hot electrons which can be transferred into an antibonding resonance of an H2 molecule adsorbed on the Au nanoparticle surface, triggering dissociation. This process is probed by detecting the formation of HD molecules from the dissociations of H2 and D2. The rate of dissociation was also profoundly dependent on of intensity and wavelength of excitation light. This work demonstrates an important application of plasmonics in the field of heterogeneous photocatalysis opening up a new pathway for all optical control of chemical reactions on metallic catalysts.