  Alice gives Bob an unknown localized physical state at some point P. At some
point Q in the causal future of P, Alice will ask Bob for the state back. Bob
knows this, but does not know at which point Q until the request is made. Bob
can satisfy Alice's summons, with arbitrarily short delay, for a quantum state
in Galilean space-time or a classical state in Minkowski space-time. However,
given an unknown quantum state in Minkowski space-time, he cannot generally
fulfil her summons. This {\it no-summoning theorem} is a fundamental feature
of, and intrinsic to, relativistic quantum theory. It follows from the
no-signalling principle and the no-cloning theorem, but not from either alone.
