He suggested that, in the future, rooms might contain more than 100 tab-like computers, 10 or 20 tabs and 1 or 2 boards, observing that "Hundreds of computers in a room could seem intimidating at first, just as hundreds of volts coursing through wires in the walls once did." Since then his vision of so many embedded computer systems appears to be correct, with many systems throughout the home now being microprocessor-controlled, even down to the toaster. This paper introduces the microprocessor, communicating and sensing technologies relevant to wearable computing. It reviews the trends and challenges that form part of the evolution of computer technology, from a computer filling a room to a room full of invisible computers. Not all wearable computing systems require the same level of computing performance or functionality. A Processor Performance and Flexibility of Function (PPaFF) scale is introduced to classify wearable computing systems and is related to the prototypes described in Weiser's vision of the 21 st century computer. He also anticipated the evolution of terabyte storage solutions and the development of more compact "micro-kernel" operating systems running software that could be readily added and removed. Considering wireless technologies, he observed that "Present technologies would require a mobile device to have three different network connections: tiny-range wireless, long-range wireless and very high speed wired. A single kind of network connection that can somehow serve all three functions has yet to be invented." Again, we have seen much development here also, with the development of Bluetooth and Wi-Fi wireless solutions.