Quantum information is at present a very active field of research, one in which quantum foundational issues continually arise. One reason for current interest is the hope of building quantum computers that are capable, assuming they can be made to work properly, of carrying out certain information-processing tasks faster and more efficiently than is possible using ordinary classical computers. To be sure, if the entire world is quantum mechanical then our current “classical” computers must in some real sense be quantum computers. So what is the difference? At least to a first approximation, classical computation is that kind where a single quasiclassical framework, section 6, suffices for describing the carriers of information (e.g., on-off states of transistors) and the way information is processed, whereas quantum computation is what cannot be so described. Further clarifying this distinction is one of the tasks of quantum information theory.