The invention concerns a method for monitoring a surrounding area (6) of a vehicle-trailer combination (1) formed by a motor vehicle (2) and a trailer (3), with which at least one radar sensor (7) is disposed on a rear area (8) at the trailer (3) for acquiring radar sensor data from the surrounding area (6) behind the trailer (3) and the acquired radar sensor data are transmitted to a controller (10) of the motor vehicle (2). Moreover, the invention concerns a monitoring device (5), a driver assistance system and a vehicle-trailer combination (1).