the invention relates to a device for welding profiled parts , in particular plastics profiled parts , having at least one welding head that can be positioned in a predetermined or predeterminable target welding position relative to another welding head and / or relative to a holding device . according to the invention the welding head and / or a part of the welding head , that comprises at least one heating element , such as a heating plate , for melting the profiled parts , can be positioned and / or is movable in accordance with at least one dimensional variation of at least one of the profiled parts from a target dimension in a spatial tolerance range around the target welding position .