A method of determining a position of a hand-held device with respect to a subject, wherein the hand-held device is for use on a body of the subject, the method comprising: receiving images from an imaging unit, wherein the imaging unit is arranged in or on the hand-held device; receiving a displacement signal from a displacement sensor, wherein the displacement sensor is arranged in or on the hand-held device to measure displacement of the hand-held device along the body when the hand-held device is in contact with the body; processing the received images to determine whether a body part of the subject can be identified in the received images; determining whether the hand-held device is in contact with the body; determining a mode of operation to use to determine a position of the hand-held device based on whether a body part can be identified and whether the hand-held device is in contact with the body; wherein the mode of operation to use is determined as (i) a first mode when a body part can be identified, (ii) a second mode when a body part cannot be identified and the hand-held device is not in contact with the body, and (iii) a third mode when the hand-held device is in contact with the body; and determining the position of the hand-held device with respect to the body of the subject using the received images and/or received displacement signal according to the determined mode of operation; wherein, when operating in the first mode, the step of determining the position of the hand-held device comprises processing the received images to determine a first position of the hand-held device with respect to the identified body part; wherein, when operating in the second mode, the step of determining the position of the hand-held device comprises processing the received images to determine an amount of movement of the hand-held device from the determined first position, and determining a second position of the hand-held device with respect to the identified body part from the determined first position and the determined amount of movement; and wherein, when operating in the third mode, the step of determining the position of the hand-held device comprises processing the received displacement signal to determine an amount of displacement of the hand-held device along the body from the determined second position, and determining a third position of the hand-held device with respect to the identified body part from the determined second position and the determined amount of displacement. receiving images from an imaging unit, wherein the imaging unit is arranged in or on the hand-held device; receiving a displacement signal from a displacement sensor, wherein the displacement sensor is arranged in or on the hand-held device to measure displacement of the hand-held device along the body when the hand-held device is in contact with the body; processing the received images to determine whether a body part of the subject can be identified in the received images; determining whether the hand-held device is in contact with the body; determining a mode of operation to use to determine a position of the hand-held device based on whether a body part can be identified and whether the hand-held device is in contact with the body; wherein the mode of operation to use is determined as (i) a first mode when a body part can be identified, (ii) a second mode when a body part cannot be identified and the hand-held device is not in contact with the body, and (iii) a third mode when the hand-held device is in contact with the body; and determining the position of the hand-held device with respect to the body of the subject using the received images and/or received displacement signal according to the determined mode of operation; wherein, when operating in the first mode, the step of determining the position of the hand-held device comprises processing the received images to determine a first position of the hand-held device with respect to the identified body part; wherein, when operating in the second mode, the step of determining the position of the hand-held device comprises processing the received images to determine an amount of movement of the hand-held device from the determined first position, and determining a second position of the hand-held device with respect to the identified body part from the determined first position and the determined amount of movement; and wherein, when operating in the third mode, the step of determining the position of the hand-held device comprises processing the received displacement signal to determine an amount of displacement of the hand-held device along the body from the determined second position, and determining a third position of the hand-held device with respect to the identified body part from the determined second position and the determined amount of displacement.