The invention relates to a method for measuring the depth of penetration of a laser beam (19) into a workpiece (24, 26), wherein the laser beam is focused in a focal spot (22) by means of a focusing optical unit (14) arranged in a machining head. The focal spot produces a vapor capillary (88) in the workpiece. An optical coherence tomograph (40) produces a first measurement beam (70a) and a second measurement beam (70b). The first measurement beam (70a) is directed at a first measurement point (MPa) at the base of the vapor capillary (88) in order to thereby measure a first distance (a1) between a reference point and the first measurement point (MPa). At the same time, the second measurement beam (70b) is directed at a second measurement point (MPb) on a surface (92) of the workpiece (24) which faces the machining head (14) and which is outside of the vapor capillary (88) in order to thereby measure a second distance (a2) between the reference point and the second measurement point (MPb). The depth of penetration (d) of the laser beam then results as the difference between the second distance (a2) and the first distance (a1).