The present invention relates to a method and to a monitoring device for monitoring operation of a drug delivery device, the monitoring device comprising: - at least a first and a second sensor (23, 25) arranged at a distance from each other with regard to a first direction (2, 3) and being adapted to generate a first and a second electrical signal (31, 33) in response to an operation of the device, - a processing unit (43, 44) configured to determine a time delay (36) between the first and the second electrical signals (31, 33) and being adapted to determine at least one state parameter of the drug delivery device on the basis of said time delay (36).
