The surface tension of a very thin film deviates from its thermodynamic thick film analogue. Generalized surface tension expressions can be obtained from intermolecular forces in order to calculate some important parameters such as the spreading coefficient of very thin films and the equilibrium shape and thickness of a pancake drop. These expressions become important for ultrathin films such as those relevant for oil extraction and for some biological calculations (in relation to vesicles and liposomes). We discuss such uses either when a thin surfactant layer covers a surface modifying its effective surface tension as a function of the surfactant layer thickness, or when a thin liquid layer is covering a surface and the thin film itself has a lower surface tension than the bulk value. Possible research directions in the field are outlined. (C) Koninklijke Brill NV, Leiden, 2011