  It is widely known that `collapse of the wave function' on a quantum system A
may be brought about by an interaction with another quantum system B. We will
prove that this is not just a possible, but a necessary consequence of
information transfer from A to B. We generalize this in order to explain why
coherences are normally not observed in macroscopic quantum systems. Finally,
we provide a quantitative insight into the balance between information gain and
state disturbance. We define the quality of an information transfer. For all
information transfers of a certain quality, we find a minimum amount of state
collapse. Along the way, we obtain generalizations of the Joint Measurement
Theorem and of the Heisenberg Principle.
