I don't want to argue about whether the notion of "wave function collapse" is a good way of understanding quantum mechanics, or not. For the purposes of this discussion, let's just adopt uncritically the naive approach to quantum mechanics, that: Between measurements, the system evolves according to Schrodinger's equation. A measurement always produces an eigenvalue of the operator corresponding to the quantity being measured. Immediately after a measurement, the wavefunction "collapses" to an eigenstate of that operator corresponding to the eigenvalue that you measured. My question is: how is the entropy of a system affected by measurement? There is a sense in which it acts like a random perturbation, and so I would think that it would increase the entropy, but on the other hand, the state becomes more definite after a measurement, which would make me think that the entropy has been lowered. Does my question make any sense, and if so, does it have a standard answer?