Observations and measurements are at the basis for all experimental activity, and for the development and application of physical theories. In quantum physics, measurements have a very special role, as fundamental principles make it impossible to predict their precise outcome, while they dictate a disturbance, or back action, on the physical system as consequence of the measurement process.
To understand the precision limits of quantum clocks and sensors, one must take this randomness and disturbances into account. However, sometimes these effects can also be a resource and an effective way to control the evolution of a quantum system, since measuring a certain property is a way to prepare a physical system to possess that property. In complex systems with many particles, measurements may thus play a role supplementary to interactions, and they may at the same time witness and induce collective properties such as squeezing and entanglement, or superradiance and phase transitions. The theoretical description of quantum measurements is well established for simple systems, while applications to many-body physics pose open questions, which will be explored within CCQ.