An interface is the region that separates two bulk phases. An interface is a molecular structure, given that one of its characteristic lengths is of molecular size. It is therefore non trivial to precisely define the intrinsic surface, the mathematical surface that describes the interface at the microscopic level. The cost in terms of the Helmholtz energy to create such a structure is known as the the surface tension. The simplest, most studied, and perhaps most important interface is the fluid/fluid interface: the liquid/vapour interface of a one-component system.