This thesis presents an experimental and theoretical study of the optical properties for two distinct planer metallic structures: metallodielectric gratings with subwavelength slots and metal nanoshells above a conducting plane. For metallodielectric gratings, two types of anomalies are present in the spectra: an edge anomaly associated with the Rayleigh wavelength, and a resonant anomaly associated with the excitations of surface plasmons. The zeroth-order transmission and reflection were measured to determine the spectral location of these anomalies and their dispersion relationships. The experimental data is compared to theoretical curves calculated using a surface impedance boundary condition approximation. The surface plasmons exhibit an energy gap in their dispersion, which is sensitive to the dielectric properties of the surrounding media. The surrounding media is changed by attaching a second grating to form a crossed grating structure, submerging the gratings in a variety of solvents, or chemically functionalizing the grating. In the first two cases, the plasmon dispersion is shifted to lower energies, the plasmon travel at a slower group velocity, and smaller energy gap is measured. The response of the plasmon dispersion to chemical functionalization is identical, except that the energy gap is increased. The difference in this trend is explained by comparing plasmons traveling on periodic structures to electrons traveling in a periodic potential. The optical properties of metal nanoshells above a conducting plane are also investigated. When a nanoshell is positioned close to a conducting plane, the surface electrons of the plane will arrange themselves to mimic the electromagnetic field of the nanoshell and its mirror image. This interaction between a nanoshell and its image plasmon approximates a nanoshell dimmer. Transmission spectra are measured as a function of the angle of polarization and compared to the expected spectra of a nanoshell dimer. The thickness of the conducting plane is also varied, which leads to a blue shift in the plasmon resonances. This shift in energy is qualitatively explained by explained using a plasmon hybridization model.