Abstract: A mobile device case system includes a first case system component usable to at least partially enclose a mobile device. A second case system component is attachable to the first case component (e.g., mechanically and/or electronically) to provide additional features and functionality such as, for example, an expansion battery and/or an external display. The second case system component can be replaced with other system components to, for example, change the display, replace the battery, or provide a decorative cover for the external surface of the case system.