Abstract: Head-mountable devices can include connection mechanisms that provide adjustable and exchangeable connections with other devices to enhance performance of the head-mountable device. Such connections can provide both mechanical engagement and operable communication between the connected devices. Accessory devices and/or external devices can be easily connected to provide different components and functions at different times as desired. Accordingly, a main portion of the head-mountable device need not include permanent components that provide every function that will later be desired by the user. Instead, the head-mountable device can have expanded and customizable capabilities by the use of one or more accessory devices.