  High redshift radio galaxies are great cosmological tools for pinpointing the
most massive objects in the early Universe: massive forming galaxies, active
super--massive black holes and proto--clusters. We report on deep narrow--band
imaging and spectroscopic observations of several z > 2 radio galaxy fields to
investigate the nature of giant Ly-alpha nebulae centered on the galaxies and
to search for over--dense regions around them. We discuss the possible
implications for our understanding of the formation and evolution of massive
galaxies and galaxy clusters.
