  Recent experimental results point to the existence of coherent quantum
phenomena in systems made of a large number of particles, despite the fact that
for many-body systems the presence of decoherence is hardly negligible and
emerging classicality is expected. This behaviour hinges on collective
observables, named quantum fluctuations, that retain a quantum character even
in the thermodynamic limit: they provide useful tools for studying properties
of many-body systems at the mesoscopic level, in between the quantum
microscopic scale and the classical macroscopic one. We hereby present the
general theory of quantum fluctuations in mesoscopic systems and study their
dynamics in a quantum open system setting, taking into account the unavoidable
effects of dissipation and noise induced by the external environment. As in the
case of microscopic systems, decoherence is not always the only dominating
effect at the mesoscopic scale: certain type of environments can provide means
for entangling collective fluctuations through a purely noisy mechanism.
