Molecular structures of interfacial proteins and peptides play important roles in biocompatibility, anti-biofouling control, membrane protein function, and anti-microbial peptide behavior. Sum frequency generation (SFG) vibrational spectroscopy has been applied to investigate molecular structures of proteins and peptides at the solid/liquid interface in situ. A thin film model was adopted to interpret SFG spectra. Isotope labeled method was introduced in the SFG studies. SFG results demonstrate that C-H stretching and amide signals can elucidate structures of hydrophobic side chains and secondary structures of interfacial peptides and proteins respectively. Detailed correlations between SFG amide I signals and secondary structures of interfacial proteins and proteins have been deduced. Various peptides and proteins including several anti-microbial peptides, albumin, lysozyme, fibrinogen, and factor XII (FXII) have been studied. Time-dependent structural changes of adsorbed proteins at the solid/liquid interface have been monitored. In addition, SFG chiral spectra were collected from interfacial peptides and proteins, which provide additional measurements to determine structures of these proteins and peptides. We also showed that combinations of linear vibrational spectroscopy such as attenuated total reflection Fourier transform infrared spectroscopy with nonlinear vibrational spectroscopic techniques such as SFG and four-wave mixing provide more detailed structure of surfaces/interfaces than that from a single technique.