Abstract: A method for measuring the penetration depth of a laser beam into a work-piece. A focusing optical unit arranged in a machining head focuses the laser beam in a focal spot. The focal spot produces a vapor capillary in the workpiece. An optical coherence tomograph produces a first and a second measurement beam. The first measurement beam is directed at a first measurement point at the base of the vapor capillary in order to thereby measure a first distance between a reference point and the first measurement point. At the same time, the second measurement beam is directed at a second measurement point on a surface of the workpiece which faces the machining head and which is outside of the vapor capillary measuring a second distance between the reference point and the second measurement point. The depth of penetration of the laser beam is the difference between the second and the first distances.