As HCI moves off the desktop and into the un-tethered world, the familiar physical elements of the interface must give way to new styles of interaction using a new class of devices. And of course, designers must confront a whole new class of physical constraints.

Un-tethered UI is as much about power and bandwidth as about psychology, visual design, and ergonomics. Microsoft Research's hardware devices group has been experimenting with small, wearable devices that sense user state and transmit it to the computing environment without consuming much power or bandwidth. This talk is an overview of the experiments and an explanation of the constraints for un-tethered interaction.