The ripples and waves on a liquid-vapour interface persist down to the scale of individual molecules. To understand the fluid dynamics of this interface, we use Molecular Dynamics (MD), the most fundamental classical model of a fluid. Using MD, we can observe the formation of a liquid phase and vapour phase by simply solving Newton’s laws. Quantities like surface tension and viscosity are outputs of these simulations. The problem then becomes one of extracting the fluid dynamics, as we know them in a continuum or experimental sense, from this molecular model. In this talk I present recent work on a mathematical framework to do this, defining a control volume in a Lagrangian reference frame moving with the surface. From this, we can pick apart the interface itself, exposing the molecular structure and quantifying the contributions to surface tension both through and along the interface. I finish by discussing potential future applications of this technique including measuring Marangoni flows, understanding static and dynamic contact lines as well as exploring boiling and evaporation.