  Sufficiently inclusive processes, like the deep inelastic scattering (DIS),
are described in terms of scale-dependent parton distributions, which
correspond to the density of partons with a given longitudinal momentum
fraction, integrated over the parton transverse momentum. For less inclusive
processes, one needs to consider densities unintegrated over the transverse
momentum. This work focuses on the unintegrated gluon distribution (UGD),
describing the probability that a gluon can be emitted by a colliding proton,
with definite longitudinal fraction and transverse momentum. Through the
leptoproduction of the $\rho$-meson at HERA, existent models for the UGD will
be investigated and compared with experimental data.
