Mobile computing platforms combining small, lightweight, low-power devices with wireless network connectivity enable the performance of familiar tasks in new environments and create opportunities for novel interactions. Since mobility imposes significant cognitive and ergonomic constraints affecting device and application usability, ease of use is central to devices in the fully mobile wirelessly connected (FMWC) world. In this paper, we consider mobility as an attribute both of the computer and the user. We explain the differences between transportable and fully mobile devices, and we contrast applications that are essentially FMWC applications, those that can be adapted to the FMWC context, and those that are unsuitable for it. We discuss the unique challenges to usability for mobile users and devices and their interaction, and we point out the increasingly critical role of usability in the mobile environment.