A method for joining component parts, wherein: a) a holding device (1; 31) holds a first component part; b) a positioning robot (2; 2, 5) positions a second component part in a joining position relative to the first component part and presses it against the first component part in the joining position; c) the component parts (10, 20) are tensed against each other in the joining position by contact between the component parts (10, 20) only; d) and the component parts (10, 20) are joined in the joining position which is determined only by the component parts' contact relative to each other, wherein e1) the first component part is provided with a first mark (11; 12; 13; 14) at a marking location, and the second component part is provided with a second mark (21; 22; 23; 24) at a marking location which is preferably formed congruently with the marking location of the first component part; e2) the marks (11, 21; 12, 22; 13, 23; 14, 24) are detected by means of a sensor when positioning the component parts (10, 20); e3) the position which the first mark (11; 12; 13; 14) assumes relative to the second mark (21; 22; 23; 24) is ascertained; e4) the robot or robots (1; 1, 2; 2, 5) positions or position the component parts (10, 20) in the joining position in accordance with the ascertained relative position of the marks (11, 21; 12, 22; 13, 23; 14, 24), wherein e5) the marking locations are brought into contact with each other in the joining position, e6) and the position which the first mark (11; 12; 13; 14) assumes relative to the second mark (21; 22; 23; 24) is detected by sensor and ascertained while the marking locations are in contact. a) a holding device (1; 31) holds a first component part; b) a positioning robot (2; 2, 5) positions a second component part in a joining position relative to the first component part and presses it against the first component part in the joining position; c) the component parts (10, 20) are tensed against each other in the joining position by contact between the component parts (10, 20) only; d) and the component parts (10, 20) are joined in the joining position which is determined only by the component parts' contact relative to each other, wherein e1) the first component part is provided with a first mark (11; 12; 13; 14) at a marking location, and the second component part is provided with a second mark (21; 22; 23; 24) at a marking location which is preferably formed congruently with the marking location of the first component part; e2) the marks (11, 21; 12, 22; 13, 23; 14, 24) are detected by means of a sensor when positioning the component parts (10, 20); e3) the position which the first mark (11; 12; 13; 14) assumes relative to the second mark (21; 22; 23; 24) is ascertained; e4) the robot or robots (1; 1, 2; 2, 5) positions or position the component parts (10, 20) in the joining position in accordance with the ascertained relative position of the marks (11, 21; 12, 22; 13, 23; 14, 24), wherein e5) the marking locations are brought into contact with each other in the joining position, e6) and the position which the first mark (11; 12; 13; 14) assumes relative to the second mark (21; 22; 23; 24) is detected by sensor and ascertained while the marking locations are in contact.