A double-screw extruder for thermoplastic materials, with a first and a second extruder screw, whereby the extruder screws have an inlet and an outlet and a core, helically extending screw flights being arranged through the core from the inlet to the outlet, so that the screw peaks and valleys are circumferential, and the peaks of the first screw penetrate into the valleys of the second screw, and vice versa. The screw flights may have a geometry which is step-shaped in cross section, whereby the steps may be provided such that the stepped course of the peaks is congruent with the stepped course of the valleys.