A supplemental device for attachment to an injection device, the supplemental device comprising: an aligning arrangement (53, 54; 71) for ensuring a predetermined positional relationship between the supplemental device and the injection device; and a securing arrangement (51-56; 63-68) for securing the supplemental device to the injection device, wherein there is an optical reading arrangement (25, 29) directed at and at least partially covering a display of the injection device when the predetermined positional relationship between the supplemental device and the injection device is present, wherein the aligning arrangement comprises first and second protuberances (53, 54) located on first and second support members (55, 56) respectively, the supplemental device being configured such that the protuberances engage in indents on the injection device when the predetermined positional relationship is present, wherein the supplemental device has a main body defining an injection device receiving channel, wherein the protuberances are located on either side of the injection device receiving channel, and wherein the support members are biased towards each other, wherein the securing arrangement comprises a closure that is attachable across the injection device receiving channel, such that movement of the injection device relative to the supplemental device in a direction orthogonal to the longitudinal axis of the injection device receiving channel is inhibited, and wherein the protuberances are configured such that, when they engage in the indents on the injection device when the predetermined positional relationship is present, movement of the supplemental device along the longitudinal axis of the injection device receiving channel is inhibited. an aligning arrangement (53, 54; 71) for ensuring a predetermined positional relationship between the supplemental device and the injection device; and a securing arrangement (51-56; 63-68) for securing the supplemental device to the injection device, wherein there is an optical reading arrangement (25, 29) directed at and at least partially covering a display of the injection device when the predetermined positional relationship between the supplemental device and the injection device is present, wherein the aligning arrangement comprises first and second protuberances (53, 54) located on first and second support members (55, 56) respectively, the supplemental device being configured such that the protuberances engage in indents on the injection device when the predetermined positional relationship is present, wherein the supplemental device has a main body defining an injection device receiving channel, wherein the protuberances are located on either side of the injection device receiving channel, and wherein the support members are biased towards each other, wherein the securing arrangement comprises a closure that is attachable across the injection device receiving channel, such that movement of the injection device relative to the supplemental device in a direction orthogonal to the longitudinal axis of the injection device receiving channel is inhibited, and wherein the protuberances are configured such that, when they engage in the indents on the injection device when the predetermined positional relationship is present, movement of the supplemental device along the longitudinal axis of the injection device receiving channel is inhibited.