The Universe’s largest galaxy clusters likely built the majority of their massive >10^11 M⊙ galaxies in simultaneous, short-lived bursts of activity well before virialization. The most challenging observational hurdle in identifying such pre-virialized “protoclusters” is their very large volumes, ~10^4 comoving Mpc^3 at z > 2, subtending areas ~half a degree on the sky. Thus the contrast afforded by an overabundance of very rare galaxies in comparison to the background can more easily distinguish overdense structures from the surrounding, normal density field. There are now five well-known 2 < z < 3 proto-clusters from the literature which are found to contain up to 12 dusty starbursts or luminous AGN galaxies each, a phenomenon that is unlikely to occur by chance even in overdense environments. I will discuss these in addition to some higher-redshift (4 < z < 5.5) groups, whose evolutionary fate is less clear. Measurements of DSFGs’ gas depletion times suggest that they are indeed short-lived on ~100 Myr timescales, and accordingly the probability of finding a structure containing more than 8 such systems is 0.2%, unless their ‘triggering’ is correlated on very large spatial scales, ~10 Mpc across. The volume density of DSFG-rich protoclusters is found to be comparable to all >10^15 M⊙ galaxy clusters in the nearby Universe, a factor of five larger than expected in some simulations. Some tension yet exists between measurements and simulations. However, improved observations of protoclusters over large regions of sky will certainly shed more light on the assembly of galaxy clusters, thus fundamental parameters governing cosmology, and also the role of environment in shaping the formation and evolution of galaxies.