Setting or fastening method for fastening an undercut anchor (2; 30) with a substantially hollow cylindrical body extending along and around a longitudinal anchor axis (2.7; 30.7), which extends from an outer end arranged in the installation position to an inner end, the outer end having a radially outwardly projecting, seating shoulder (2.1; 30.1), which has a through opening (2.3; 30.3) extending along the longitudinal axis of the anchor and having an internal thread, which is designed to be placed around a blind hole on an outer surface of a joining partner and comprising wrench flats (2.2; 30.2) on an outer circumferential surface for rotationally fixed connection to a tool, in particular a setting tool, wherein a plurality of arm-shaped and spreadable spreading arms (2.4; 30.4) extending parallel offset to the longitudinal axis of the anchor are formed by the seating shoulder (2.1; 30.1), and wherein an undercut shoulder (2.5; 30.5) projecting radially outwardly is formed at the inner end of each of the spreader arms (2.4; 30.4), in a joining partner, especially a concrete or composite slab, comprising the method steps: drilling a blind hole in the joining partner, which extends from a blind hole upper end at an outer surface of the joining partner to a blind hole lower end in the joining partner, and forming an undercut in the joining partner at the bottom of the blind hole, CHARACTERIZED IN that the undercut is formed only by rotating and spreading the undercut anchor (2; 30). which extends from an outer end arranged in the installation position to an inner end, the outer end having a radially outwardly projecting, seating shoulder (2.1; 30.1), which has a through opening (2.3; 30.3) extending along the longitudinal axis of the anchor and having an internal thread, which is designed to be placed around a blind hole on an outer surface of a joining partner and comprising wrench flats (2.2; 30.2) on an outer circumferential surface for rotationally fixed connection to a tool, in particular a setting tool, wherein a plurality of arm-shaped and spreadable spreading arms (2.4; 30.4) extending parallel offset to the longitudinal axis of the anchor are formed by the seating shoulder (2.1; 30.1), and wherein an undercut shoulder (2.5; 30.5) projecting radially outwardly is formed at the inner end of each of the spreader arms (2.4; 30.4), in a joining partner, especially a concrete or composite slab, comprising the method steps: drilling a blind hole in the joining partner, which extends from a blind hole upper end at an outer surface of the joining partner to a blind hole lower end in the joining partner, and forming an undercut in the joining partner at the bottom of the blind hole, CHARACTERIZED IN that the undercut is formed only by rotating and spreading the undercut anchor (2; 30). drilling a blind hole in the joining partner, which extends from a blind hole upper end at an outer surface of the joining partner to a blind hole lower end in the joining partner, and forming an undercut in the joining partner at the bottom of the blind hole, CHARACTERIZED IN that the undercut is formed only by rotating and spreading the undercut anchor (2; 30).