  Between 1992 and 2007, the HERA accelerator provided $ep$ collisions at
center of mass energies beyond $300 \ {\rm GeV}$ at the interaction points of
the H1 and ZEUS experiments. Interesting results to emerge relate to the newly
accessed field of perturbative strong interaction physics at low Bjorken-$x$,
where parton densities become extremely large. Questions arise as to how and
where non-linear dynamics tame the parton density growth and challenging
features such as geometric scaling are observed. Central to this low $x$
physics landscape is a high rate of diffractive processes, in which a colorless
exchange takes place and the proton remains intact. A review is given for main
results obtained by H1 and ZEUS experiments in this field.
