The two important discretization methods in TCAD are the box integration method (finite volume or control volume method) and the finite element method. Each method imposes certain requirements on the mesh. For the box integration method they result in conditions for the triangles of the surface mesh. For the finite element method geometrical criteria, as discussed in the previous section, suffice for most applications. However, for diffusion problems which are important in semiconductor process simulation further principles need to be investigated. A few simple examples will be presented to show the influence of different meshes on the validity of the discretization.