Summary: Modern communication theory originated from the attempts of communication engineers to understand what they were doing, in the most general terms. When the mathematical concept of information had crystallized out of these attempts, it was found that a track beaten by theoretical physicists led to the same idea, and, as usual, it was found that pure mathematicians had prepared the way to even more general developments. The new science of information theory thus connects several fields of research, old and new, each with its own techniques.