Abstract: An electronic add-on module for an injection device includes a longitudinal axis and a discharge button arranged at a proximal end and configured to be movable in a distal direction along the longitudinal axis for discharging a liquid medication out of a container of the injection device. The add-on module includes a sensor unit, a receiving region adapted to the shape of the housing of the injection device for placing on along the longitudinal axis thereof, and a holding mechanism configured to releasably secure the add-on module against moving axially relative to the injection device. The receiving region is configured such that the discharge button can be accessed or contacted when the add-on module is on the injection device. The add-on module comprises a release element configured to be moved solely on a plane perpendicular to the longitudinal axis in a release movement for releasing the holding mechanism.