  The observed clustering of Lyman-$\alpha$ lines is reviewed and compared with
the clustering of CIV systems. We argue that a continuity of properties exists
between Lyman-$\alpha$ and metal systems and show that the small-scale
clustering of the absorbers is consistent with a scenario of gravitationally
induced correlations. At large scales statistically significant over and
under-densities (including voids) are found on scales of tens of Mpc.
