A number of recent experiments in mesoscopic physics have raised anew the question of what constitutes an “ideal” quantum detector, that is a detector which produces a minimal disturbance of the system being probed. I will discuss recent theoretical work addressing this issue, taking a point of view which stresses the physics of mesoscopic noise, and its connection to ideas from information theory. I will also discuss how these ideas apply to the quantum non-demolition measurement of a qubit, and to the study of nano-electromechanical systems.