Probing the Large Scale Structure with QSOs and radio-galaxies
  QSOs and radio-galaxies, together with the CMB, ``normal'' galaxies and clusters, represent the main source of information about the origin and evolution of the Large Scale Structure. They can be used either directly, as tracers of the density peaks, or as cosmic lighthouses, probing the Universe along the line of sight with their conspicuous flow of photons. The amplitude of the QSO clustering has been measured to be $r_o \simeq 6 \hmpc$ at $z\sim 2$ and appears to increase with redshift. These properties are consistent with a scenario in which the quasars, with a lifetime of $t \sim 10^8 yr$, sparsely sample halos of mass greater than $M_{min} \sim 10^{12}-10^{13} h^{-1}$ M$_{\odot}$. Radio-galaxies are found to cluster on similar scales. The study of QSO and radio-galaxy environments is in qualitative agreement with their clustering properties. Absorption-line systems observed in high-resolution spectra of QSOs show structure, including voids/underdensities and overdensities, from the smallest scales to tens of Mpc. QSO pairs, groups and lenses allow us to get a 3-D glimpse of the absorbers, providing invaluable information about their size, ionization, mass. Great progress in this field is expected from the new instrumentation coming into operation at the 8-m class telescopes. 
S. Cristiani