Zinc oxide is one of the most-studied and technologically relevant metal oxides and is particularly interesting because its nanoparticles are inherently photoluminescent. A variety of surface science techniques, including X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS), have been used to investigate the adsorption of molecules on zinc oxide single crystal and nanoparticle surfaces. Adsorbates that have been studied include thiols, sulfur dioxide and nitrogen dioxide, and the effects of coadsorbates, such as water, have been evaluated. The effects of adsorption on the photoluminescence spectrum of ZnO have also been investigated. Applications of this research for electronic materials, heterogeneous catalysis, and dye-sensitized solar cells are discussed.