A laser beam may be regarded as a tool when it is used to cut, weld or harden metals. It is essential to achieve optimum, constant adjustment of the laser beam parameters, e.g. focal radius, focal position, intensity distribution, etc. Within relatively narrow limits in order to increase the quality and reliability of the products manufactured in laser beam machining units. A laser unit can be set-up and monitored optimally for a machining task by means of diagnosis and monitoring of the unfocused and focused laser radiation. This form of laser characterization forms the basis for constant product quality and process reliability. The power density distribution of a laser beam can be determined and the laser beam characterized with the aid of a hollow needle measuring system which has been developed for this purpose. By using suitably matched hollow needles, it is possible to perform such measurements in the free beam, but also in the laser beam focus, enabling us to measure the beam caustic. A variety of different types of projection are available for displaying the power density distribution, ranging from isometric projection to contour line displays. A modified form of contour line displays is especially suitable for determining beam parameters, e.g. the beam radius. Further laser beam or working beam parameters, like iterative determination of the caustic, short-short-term stability of the laser, Rayleigh length, etc., can be obtained using this method of measurement.