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. Interfaces are subject to thermal noise, which leads to thermal capillary waves.