Abstract: An exchangeable cover (12, 14) for a mobile telephone (10) is configured to cover at least part of the mobile telephone. The cover itself may comprise an electronic memory (24) containing functionality or data which can be imparted to a mobile telephone core (16) via contacts (26, 28). The functionality or data contained within the electronic memory (24) is such as to influence operation of the application program embedded within the mobile telephone unit (16). In this way, at least some of the user interfaces of the mobile telephone (10) can be added to or modified by clipping on the exchangeable cover (12, 14) to the mobile telephone (10). The shape and/or patterning applied to the cover may have an association or theme in common with the functionality or data embedded within the electronic memory (24).