A collection of results on the observability of quantum mechanical systems, with an emphasis on the case of nonselective measurement, is presented in this paper. In particular, the authors use the formalism of generalized measurement and of effects and operations to extend the basic definitions and criteria of observability to the case of general measurement by introducing an effective observable. Other results include the characterization of a “best probe” (in other words, a minimally disturbing probe) for indirect measurement and an interesting study of the relation between disturbance and observability. The authors also discuss how the observability properties of a quantum system relate to the problem of state reconstruction. Finally, the authors state that extensions of their definitions and results to continuous measurements, to optimization of the methods for state determination in specific settings, and to applications of observer design in closed-loop quantum systems are just a few of the possible subjects for future research. The paper is well written and, though it is not easily apprehensible to nonspecialists, it clearly explains the concerns and the results on the observability of quantum systems. In conclusion, this is an interesting work intended for professionals.