Abstract: The invention relates to a method for ascertaining an efficiency of a drive train (1), in particular of a drive train (1) or of part of a drive train of a wind turbine (4, 7, 11), the drive train (1) having a first end (1a) with a first shaft portion (11) driveable by means of a first drive device (6) and a second end (1b) with a second shaft portion (27) driveable by means of a second drive device (7), between which there is arranged a mechanical transfer element, in particular a gear unit, the first end of the drive train (1) being driven in a first test and a variable dependent on the torque acting on the first shaft portion being detected at the first end (1a) of the drive train (1) and a variable dependent on the torque acting on the second shaft portion being detected at the second end (1b) of the drive train (1), and the second end (1b) of the drive train (1) being driven in a second test and the variable dependent on the torque acting on the second shaft portion being detected at the second end (1b) o