An assembly for a drug delivery device is provided, comprising a housing, a rotation member adapted and arranged to be rotated in a first and in a second rotational direction with respect to the housing in an operational mode of the assembly, a drive member which is configured to mechanically cooperate with the rotation member such that the drive member follows rotation of the rotation member when the rotation member is rotated in the second rotational direction and such that the rotation member rotates with respect to the drive member when the rotation member is rotated in the first rotational direction, and a piston rod. The assembly further comprises at least one biasing member which is adapted and arranged to provide a force tending to bring the rotation member and the drive member out of mechanical cooperation in the operational mode. The assembly is switchable from the operational mode into a reset mode. For switching from the operational mode into the reset mode, the biasing member is configured to drive a relative axial movement between the drive member and the rotation member such that the rotation member and the drive member are brought out of mechanical cooperation. Furthermore, a drug delivery device is provided.