The invention relates to a device for monitoring a yarn (1), which is moving in its longitudinal direction, using a measuring head and an evaluation unit (7) which is spatially separate from and connected to the measuring head. To enable optimum use of measuring heads operating according to different physical principles without unduly increasing the outlay, the measuring head includes a first part (2) operating according to a first measuring principle and having a connection (5) to a first part (6) of the evaluation unit for making an evaluation according to the first measuring principle. The measuring head then comprises a further part (9) operating according to a further measuring principle and having a releasable connection (8, 10) to the first part. In addition, the evaluation unit likewise comprises a further part connected by a releasable connection for the evaluation according to the further measuring principle.