In popular, hierarchical models the most massive galaxies and clusters of galaxies are expected to form in over-dense regions. There is considerable evidence that powerful radio galaxies are uniquely associated with the most massive galaxies (and black holes). For example, the combined near-IR `Hubble' K-z relation for radio and field galaxies shows that high-z radio galaxies (HzRGs) are among the most massive galaxies at any given epoch up to z = 5.2 (De Breuck et al 2001, astro-ph/0109540). HzRGs are therefore ideal beacons for studying the formation and evolution of massive galaxies and proto-clusters. I will present the results of ultra-deep Ly-a imaging and spectroscopy (optical and IR) of several high redshift (z > 2) radio galaxies and their surrounding fields using the Keck and VLT telescopes. The observations show giant (150 kpc) Ly-a halos centered on the radio galaxies, as well as spectroscopically confirmed over-densities of Ly-a excess galaxies at redshifts ranging from 2 to 4. I will discuss the possible origin and ionization of the halos and their filamentary structures, and the implications of the existence of proto-clusters already at redshifts up to z ~ 4.