A monitoring device for monitoring operation of a drug delivery device and to a respective drug delivery device is presented in which a fastening member releasably attaches the monitoring device to an axially elongated housing of the drug delivery device, a sliding member is shiftable relative to the fastening member in an axial direction and being adapted to operably engage with at least one dose setting component of the drug delivery device to follow an axial displacement of the dose setting component relative to the housing component, a first sensor arrangement for detecting a rotation of the dose setting component relative to the fastening member, a second sensor arrangement for detecting an axial displacement of the sliding member relative to the fastening member, and a processing member coupled to the first and to the second sensor arrangements to process first and second signals obtainable from the first and from the second sensor arrangements for determining the size of a dose dispensed by the drug delivery device.