The invention relates to a motor vehicle door lock which is equipped with a locking mechanism (1, 2) consisting of essentially a rotary latch (1) and a pawl (2), in addition to an actuating lever chain (4) and a locking unit (5, 6). Said motor vehicle door lock also comprises a security device (8, 9, 10) which can optionally set the actuating lever chain (4) and/or the locking unit (5, 6) in a non-functioning state/functioning state. According to the invention, said security device (8, 9, 10) is designed as a retrofitted module (8, 9, 10) which can be connected, according to requirements, to a base module (1-6 and 11), said module surrounding the largest part of the locking mechanism (1, 2), the actuating lever chain (4) and the locking unit (5, 6).