Method for mounting an add-on part (3, 3") onto a work piece (1, 1"), in particular onto a motor vehicle body, wherein the add-on part (3, 3") is mounted under positional precision with respect to a reference region (12, 13, 12", 13") on the work piece (1, 1 "), - in which method a mounting tool (5, 5") guided by means of a robot (14, 14") is utilized, which [tool] comprises a fixing device (19, 19") for receiving the add-on part (3, 3") and a sensor system (20, 20"), permanently connected with the mounting tool (5, 5"), with at least one sensor (21, 21"), wherein using the sensor system (20, 20") measured values are registered in select regions (12, 13, 12", 13") of the work piece (1, 1") as well as also in select regions (26, 27, 22") of the add-on part (3, 3"), - wherein, starting from an approximation position (34, 34") that is independent of the position of the work piece (1, 1") in the working space of the robot (14, 14"), the mounting tool (5, 5") with the add-on part (3, 3") held in the fixing device (19, 19") is moved into a mounting position (25, 25") in which the add-on part (3, 3") held in the mounting tool (5, 5") is oriented positionally precise with respect to the reference region (12, 13, 12", 13") of the work piece (1, 1"), - wherein for approaching the mounting position (25, 25") an iterative regulation process is passed through, in the course of which an (actual) measured value of the at least one sensor (21, 21") is generated, this (actual) measured value is compared with a (nominal) measured value generated in the course of a setup phase, by utilizing a Jacobi matrix calculated in the course of the setup phase, a displacement vector of the mounting tool (5, 5") is calculated and the mounting tool (5, 5") is displaced by this displacement vector, and - wherein the add-on part (3, 3") in this mounting position (25, 25") of the mounting tool (5, 5") is subsequently connected with the work piece (1, 1"). - in which method a mounting tool (5, 5") guided by means of a robot (14, 14") is utilized, which [tool] comprises a fixing device (19, 19") for receiving the add-on part (3, 3") and a sensor system (20, 20"), permanently connected with the mounting tool (5, 5"), with at least one sensor (21, 21"), wherein using the sensor system (20, 20") measured values are registered in select regions (12, 13, 12", 13") of the work piece (1, 1") as well as also in select regions (26, 27, 22") of the add-on part (3, 3"), - wherein, starting from an approximation position (34, 34") that is independent of the position of the work piece (1, 1") in the working space of the robot (14, 14"), the mounting tool (5, 5") with the add-on part (3, 3") held in the fixing device (19, 19") is moved into a mounting position (25, 25") in which the add-on part (3, 3") held in the mounting tool (5, 5") is oriented positionally precise with respect to the reference region (12, 13, 12", 13") of the work piece (1, 1"), - wherein for approaching the mounting position (25, 25") an iterative regulation process is passed through, in the course of which an (actual) measured value of the at least one sensor (21, 21") is generated, this (actual) measured value is compared with a (nominal) measured value generated in the course of a setup phase, by utilizing a Jacobi matrix calculated in the course of the setup phase, a displacement vector of the mounting tool (5, 5") is calculated and the mounting tool (5, 5") is displaced by this displacement vector, and - wherein the add-on part (3, 3") in this mounting position (25, 25") of the mounting tool (5, 5") is subsequently connected with the work piece (1, 1").