Sum frequency generation (SFG) vibrational spectroscopy is highly surface sensitive because the second-order nonlinear optical process is only allowed in media without inversion symmetry, such as interfaces. As an all-optical technique, SFG can be employed to study molecular compositions and structures at solid/liquid and at solid/high pressure gas interfaces. This paper discusses the applications of SFG in our laboratory to study the reaction intermediates of catalytic reactions on metal surfaces under high-pressure conditions, the surface composition and structure of polymers, and the interfacial properties of proteins on liquid/solid interfaces. [DOI: 10.1380/ejssnt.2004.106]