In this chapter wearable computers are considered from the perspective of human factors. The basic argument is that wearable computers can be considered as a form of prosthesis. In broad terms, a prosthesis could be considered in terms of replacement (i.e., for damaged limbs or organs), correction (i.e., correction to ‘normal’ vision or hearing with glasses or hearing aids), or enhancement of some capability. Wearable computers offer the potential to enhance cognitive performance and as such could act as cognitive prosthesis, rather than as a physical prosthesis. However, wearable computers research is still very much at the stage of determining how the device is to be added to the body and what capability we are enhancing.