  In Quantum Physics it is not always possible to directly perform the
measurement of an obsevable; in some of these cases, however, its value can be
{\sl detected}, i.e. it can be inferred by measuring {\sl another} observable
characterized by perfect correlation with the observable of interest. Though a
{\sl detection} is often interpreted as a {\sl measurement} of the detected
observable, we prove that the two concepts cannot be identified in Quantum
Physics. Furthermore, we establish what meaning and role can be ascribed to
detections coherently with Quantum Theory.
