This chapter begins with a mechanical description of models involving stationary and moving interfaces. The extended finite element method (X‐FEM) is then detailed for three different scenarios: crack‐like interfaces, material interfaces, and free surfaces. As in the case of related methods (generalized FEM (GFEM), partition of unity FEM (PUFEM)), X‐FEM relies on approximation technology that is based on a partition of unity construction. The method uses evolving enrichment functions to represent evolving geometric features and capture the local character of the solution in their vicinity. The use of the level‐set method to update such features is a natural complement to X‐FEM.