A supplemental device for attachment to an injection device, the supplemental device comprising: a display; a processor arrangement; a dose dialled detector operable to detect a dose of medicament dialled into an attached injection device; a dose delivery determiner for determining that a dose of medicament has been delivered; a quantity determiner for determining a quantity of medicament that has been delivered; and a clock configured to determine a current time; the processor arrangement being configured when the supplemental device is in a default mode to display both a quantity of medicament that was delivered since a last dose and a time elapsed since delivery of the last dose; and wherein the processor arrangement is configured to: respond to detection that a dose of medicament has been dialled into the attached injection device by transitioning from the default mode to a dosage dialling state wherein, in the dosage dialling state, the supplemental device determines the dose of medicament dialled into the attached injection device; subsequent to the dose delivery determiner determining that a dose of medicament has been partially delivered, detect an input indicating the delivery of a partial dose of medicament from the attached injection device, the partial dose being less than the entire dose dialled into the attached injection device, the input comprising at least one of a confirm/OK input by a user or a user turning back a dosage dialler of the injection device, and in response to detecting the input indicating the delivery of a partial dose of medicament, transition to a countdown state and, after a period of time has lapsed in the countdown state, transition to a partial dose delivered state in which the processor arrangement is caused to: display the partial dose delivered, wherein the dose delivered is equal to the dose dialled into the attached injection device minus the dose remaining; and store a value representing the partial dose delivered. a display; a processor arrangement; a dose dialled detector operable to detect a dose of medicament dialled into an attached injection device; a dose delivery determiner for determining that a dose of medicament has been delivered; a quantity determiner for determining a quantity of medicament that has been delivered; and a clock configured to determine a current time; the processor arrangement being configured when the supplemental device is in a default mode to display both a quantity of medicament that was delivered since a last dose and a time elapsed since delivery of the last dose; and wherein the processor arrangement is configured to: respond to detection that a dose of medicament has been dialled into the attached injection device by transitioning from the default mode to a dosage dialling state wherein, in the dosage dialling state, the supplemental device determines the dose of medicament dialled into the attached injection device; subsequent to the dose delivery determiner determining that a dose of medicament has been partially delivered, detect an input indicating the delivery of a partial dose of medicament from the attached injection device, the partial dose being less than the entire dose dialled into the attached injection device, the input comprising at least one of a confirm/OK input by a user or a user turning back a dosage dialler of the injection device, and in response to detecting the input indicating the delivery of a partial dose of medicament, transition to a countdown state and, after a period of time has lapsed in the countdown state, transition to a partial dose delivered state in which the processor arrangement is caused to: display the partial dose delivered, wherein the dose delivered is equal to the dose dialled into the attached injection device minus the dose remaining; and store a value representing the partial dose delivered. respond to detection that a dose of medicament has been dialled into the attached injection device by transitioning from the default mode to a dosage dialling state wherein, in the dosage dialling state, the supplemental device determines the dose of medicament dialled into the attached injection device; subsequent to the dose delivery determiner determining that a dose of medicament has been partially delivered, detect an input indicating the delivery of a partial dose of medicament from the attached injection device, the partial dose being less than the entire dose dialled into the attached injection device, the input comprising at least one of a confirm/OK input by a user or a user turning back a dosage dialler of the injection device, and in response to detecting the input indicating the delivery of a partial dose of medicament, transition to a countdown state and, after a period of time has lapsed in the countdown state, transition to a partial dose delivered state in which the processor arrangement is caused to: display the partial dose delivered, wherein the dose delivered is equal to the dose dialled into the attached injection device minus the dose remaining; and store a value representing the partial dose delivered. display the partial dose delivered, wherein the dose delivered is equal to the dose dialled into the attached injection device minus the dose remaining; and store a value representing the partial dose delivered.