Netgen is an automatic 3d tetrahedral mesh generator. It accepts input from constructive solid geometry (CSG) or boundary representation (BRep) from STL file format. The connection to a geometry kernel allows the handling of IGES and STEP files. NETGEN contains modules for mesh optimization and hierarchical mesh refinement.It uses Tcl for script binding to its C++ libraries and for GUI.Netgen is used in ngsolve, a finite element solver, and ngs-flow a computational fluid dynamics library. Both of these make use of Tcl in similar ways.