  Inclusive as well as exclusive emissions in forward and central directions of
rapidity are widely recognized as excellent channels to access the proton
structure at small-x. In this regime, to describe nucleons structure, it is
necessary to use kT-unintegrated distributions. In particular, at large
transverse momenta, the x-evolution of the so-called unintegrated gluon
distribution is driven by the Balitsky-Fadin-Kuraev-Lipatov equation, within
the framework of the high-energy factorization (HEF). Recent analyses on the
diffractive electroproduction of \rho mesons have corroborated the underlying
assumption that the small-size dipole scattering mechanism is at work, thus
validating the use of the HEF formalism. Nonetheless, a significant sensitivity
of polarized cross sections to intermediate values of the meson transverse
momenta, where, in the case of inclusive emissions, a description at the hand
of the transverse momentum dependent (TMD) factorization starts to be the most
appropriate framework, has been observed. In this work, we will review the
formal description of the UGD within the BFKL approach and present some UGD
models that have been proposed, then we will describe the state of the art of
some recent phenomenological analyses.
