Surface Evolver is an interactive program for the modelling of liquid surfaces shaped by various forces and constraints. A surface is implemented as a simplicial complex, that is, a union of triangles. The user defines an initial surface in a datafile. The Evolver evolves the surface toward minimal energy by a gradient descent method. The aim can be to find a minimal energy surface, or to model the process of evolution by mean curvature for surface tension energy in the context of varifolds and geometric measure theory.