This project entails research with the goal to extend laser cutting of steel based metals to thickness above 20 mm and laser powers in the 10 kW range, with adequate accuracy and economically viable cutting speeds. The technical approach is to develop mirror based cutting heads with truly coaxial gas jet chamber and laser beam path from the final focusing mirror. The project consists of three phases: Phase 1: Fundamental studies of cutting front mechanisms, beam propagation, nozzle design and chemical reactions in the cut kerf with special emphasize on high laser powers and thick sections. Phase 2: Development of mirror based test cutting heads. Phase 3: Test and evaluation of the cutting heads on high power CO2 laser sources in the 6-12 kW power range. The test phase concentrates on cutting steels, applied in the heavy industry. In the first part of the project fundamental studies have established the basis to determine the required power densities, Rayleigh lengths, desired gas flow and nozzle shapes. A four mirror optical system (Multi-Mirror Cutting Head) for the prototype has been designed, which honours the demands for a good focusing quality and small nozzle diameter. A coaxial cutting nozzle which can be adjusted independently to the laser beam has been developed. The position of the focus relative the workpiece can be adjusted to cutting applications with relatively large processing windows, i.e. both mild and stainless steels, and of a broad thickness range. A build-in auto-focus device which is necessary for cutting of large workpieces has also been installed. The designed and manufactured multi-mirror cutting head has been tested under various process conditions by use of interfaces for the laser system at DTU. A compact cutting head based on a two-mirror system has been developed with focus on process capabilities, easy handling during set-up and alignment, and lower weight. The cutting results show that it is possible to increase the cutting speed considerably in mild steel. In 10-13 mm mild steel the increase is over 60%. In 10 mm stainless steel the cutting results indicate that higher cutting speed also are possible, but the results are not as clear. In 20 mm mild steel the increase in the cutting rate is over 50 %. Preparations for exploitation are pt. being made and opportunities to initiate work under some of the commission's programmes concerning development and exploitation, and within the current consortium is under consideration.