"The primary focus of this thesis is the characterization of gold and silicon nanoparticles using second-order nonlinear optical spectroscopy. Nonlinear optical measurements are non-invasive and can provide information about surface and interface regions of materials. Nanoparticles have a very high surface to volume ratio, and second-order nonlinear optical phenomena are often produced from surface contributions, making them ideal methods for characterizing these nanostructures. The two methods featured in this work are second harmonic generation (SHG) and sum frequency generation (SFG) using the two beam, cross-polarized SHG/ SFG (XP2SHG/SFG) technique. Optical characterization of nanoparticles and interfaces is currently a relevant topic in solid state and nano-scale physics. A non-destructive method for characterizing nano-materials is highly desirable and the XP2SHG/SFG technique is still relatively new for these types of materials."