The main biophysical activity of surfactant is to reduce drastically the surface tension of the thin aqueous layer lining the respiratory epithelium of lungs (for a recent historical perspective see [1]). This remarkable material has a unique composition consisting of about 90% lipids, mostly phospholipids, and 8^ 10% surfactant-associated proteins. The components that are mainly responsible for the surface properties of surfactant are its phospholipids, surface tension (Q) being lowered by a mainly lipid¢ lm, traditionally considered to be monomolecularly thick, at the air^ water interface of the alveoli. Some experiments suggest that this¢ lm might be contiguous with an underlying multilayer (see chapter by Schu «rch and colleagues for a more extensive discussion). Dipalmitoylphosphatidylcholine (DPPC) is the major surfactant phospholipid, and is also its principal tensoactive compound (agent responsible for attaining very low surface tension). It is now clear, however, that speci¢ c surfactant-associated proteins are critical to impart to surfactant phospholipids dynamic properties necessary for full functionality. The main structural features of the surfactant proteins that are of interest to in the interfacial activity of surfactant are noted in Table 1. The physical properties and the role of surfactant proteins that will be discussed in this review will be those that are known to, or likely to, have direct implication in the events occurring at or around the air^ liquid interface of the respiratory spaces.