Head-mounted display systems and methods of operation that allow users to couple and decouple a portable electronic device such as a handheld portable electronic device with a separate head-mounted device (e.g., temporarily integrates the separate devices into a single unit) are disclosed. The portable electronic may be physically coupled to the head-mounted device such that the portable electronic device can be worn on the user's head. The portable electronic device may be operatively coupled to the head-mounted device such that the portable electronic device and head mounted device can communicate and operate with one another. Each device may be allowed to extend its features and/or services to the other device for the purpose of enhancing, increasing and/or eliminating redundant functions between the head-mounted de ice and the portable electronic device.