A method of pairing a combination of a drug delivery device (500, 800) and an add-on device (600, 900) with an external control device, comprising the steps of: - providing a drug delivery device (500, 800) comprising a first visual identifier (505, 805), - providing an add-on device (600, 900), comprising: - communication means allowing the add-on device to communicate with an external control device, and - a second visual identifier (605, 905), - wherein the add-on device is adapted to be releasably mounted on and in engagement with the drug delivery device, and - wherein the add-on device is adapted to capture dose related data from a drug delivery device when mounted thereon, - providing an external control device comprising: - processor means, - storage means, and - communication means allowing the external control device to communicate with the add-on device, - mounting the add-on device on the drug delivery device, characterized in that - the add-on device is adapted to receive a pairing confirmation from the external control device, whereby the add-on device is actuated from an un-paired to a paired mode, and - the external control device comprises: - image capturing means, - the storage means comprising information in respect of at least one predefined combination of a first and a second visual identifier, the method of pairing a combination of a drug delivery device (500, 800) and an add-on device (600, 900) with an external control device comprises the further steps of: - capturing an image of the add-on device mounted on the drug delivery device with the external control device, the image comprising both the first and the second visual identifier, - processing the captured image to (i) identify the first and the second visual identifier, and (ii) determine if the captured identifiers represent a predefined combination of visual identifiers, and - if it is determined that the captured identifiers represent a predefined combination, transmitting a paring confirmation from the external control device to the add-on device, thereby actuating the add-on device from the un-paired to the paired mode. - providing a drug delivery device (500, 800) comprising a first visual identifier (505, 805), - providing an add-on device (600, 900), comprising: - communication means allowing the add-on device to communicate with an external control device, and - a second visual identifier (605, 905), - wherein the add-on device is adapted to be releasably mounted on and in engagement with the drug delivery device, and - wherein the add-on device is adapted to capture dose related data from a drug delivery device when mounted thereon, - communication means allowing the add-on device to communicate with an external control device, and - a second visual identifier (605, 905), - wherein the add-on device is adapted to be releasably mounted on and in engagement with the drug delivery device, and - wherein the add-on device is adapted to capture dose related data from a drug delivery device when mounted thereon, - providing an external control device comprising: - processor means, - storage means, and - communication means allowing the external control device to communicate with the add-on device, - processor means, - storage means, and - communication means allowing the external control device to communicate with the add-on device, - mounting the add-on device on the drug delivery device, - the add-on device is adapted to receive a pairing confirmation from the external control device, whereby the add-on device is actuated from an un-paired to a paired mode, and - the external control device comprises: - image capturing means, - the storage means comprising information in respect of at least one predefined combination of a first and a second visual identifier, - image capturing means, - the storage means comprising information in respect of at least one predefined combination of a first and a second visual identifier, - capturing an image of the add-on device mounted on the drug delivery device with the external control device, the image comprising both the first and the second visual identifier, - processing the captured image to (i) identify the first and the second visual identifier, and (ii) determine if the captured identifiers represent a predefined combination of visual identifiers, and - if it is determined that the captured identifiers represent a predefined combination, transmitting a paring confirmation from the external control device to the add-on device, thereby actuating the add-on device from the un-paired to the paired mode.