  In this review we present the potentialities and the achievements of the use
of non-classical photon number correlations in twin beams (TWB) states for many
applications, ranging from imaging to metrology. Photon number correlations in
the quantum regime are easy to be produced and are rather robust against
unavoidable experimental losses, and noise in some cases, if compared to the
entanglement, where loosing one photon can completely compromise the state and
its exploitable advantage. Here, we will focus on quantum enhanced protocols in
which only phase-insensitive intensity measurements (photon number counting)
are performed, which allow probing transmission/absorption properties of a
system, leading for example to innovative target detection schemes in a strong
background. In this framework, one of the advantages is that the sources
experimentally available emit a wide number of pairwise correlated modes, which
can be intercepted and exploited separately, for example by many pixels of a
camera, providing a parallelism, essential in several applications, like wide
field sub-shot-noise imaging and quantum enhanced ghost imaging. Finally,
non-classical correlation enables new possibilities in quantum radiometry, e.g.
the possibility of absolute calibration of a spatial resolving detector from
the on-off- single photon regime to the linear regime, in the same setup.
