Software supported process planning and quality assurance are of primary importance for the realization of a manufacturing method in laser beam welding. A novel software tool for process planning and prediction of the properties of laser welded seams named CALAS, i.e. computer aided lasering, is presented. Its structure and performance are described in detail. The software tool connects a physical model of welding by carbon-dioxide laser radiation to an interactive man/machine-interface by simulation and visualization. The model considers five fundamental physical processes, absorption of laser radiation at the keyhole surface including multiple reflexion, heat conduction in liquid and solid phase, melt flow, gas flow and heat conduction in the vapor phase, and absorption of laser radiation inside the keyhole volume (plasma absorption). The effect of processing parameters and the resulting geometry of the welded seam are calculated and displayed. The interface provides a three dimensional visualization of the keyhole and the melt pool. Cross sectional views and longitudinal sections may be displayed two-dimensional. Besides typical parameters the beam characteristics, such as power density distribution, and material properties, such as temperature dependent surface tension and heat conductivity, are included in the calculation too. Due to direct and fast prediction of process parameters and seam geometry CALAS serves as a controlling tool for the operator at the machine tool, and in a more general view, it serves as a tool for processing planning and quality assurance in industry.