  The engine that powers quantum cryptography is the principle that there are
no physical means for gathering information about the identity of a quantum
system's state (when it is known to be prepared in one of a set of
nonorthogonal states) without disturbing the system in a statistically
detectable way. This situation is often mistakenly described as a consequence
of the ``Heisenberg uncertainty principle.'' A more accurate account is that it
is a unique feature of quantum phenomena that rests ultimately on the Hilbert
space structure of the theory along with the fact that time evolutions for
isolated systems are unitary. In this paper we shall explore several aspects of
the information--disturbance principle in an attempt to make it firmly
quantitative and flesh out its significance for quantum theory as a whole.
