The many opportunities offered by wearable computing have triggered the imaginations of designers and researchers in a wide variety of fields. The inevitability of computers and interfaces that are small enough to be worn on the human body has inspired the creation of devices and applications that can assist with specialized professional and personal activities, as well as aid and augment everyday life in the modern world. In reality, limitations imposed by factors such as battery life, processor power, display brightness, network coverage and form have conspired to delay the widespread introduction of wearable computers. Nevertheless, over the past ten years there have been many successful implementations, and, as the relentless miniaturization of computing devices continues, an increasing number of viable applications are emerging. In this chapter, a generic wearable computer architecture is outlined and its application to commercial and research designs presented. Applications are reviewed from early aircraft maintenance and military designs; designs for personal assistance, communication, and health monitoring; and prototype implementations for real-world gaming and smart fashion textiles. The challenges presented by these applications, including technical limitations, user interface and system design, and social issues are identified and discussed.