Abstract: Galaxy clusters serve as unique laboratories to study galaxy formation and cosmology. Nevertheless, little is known about the early stage of cluster formation when the cluster members assembled the bulk of their masses. Long before these galaxies are observed, they shut down star formation and evolved passively since. While this general picture is accepted, the detailed star formation histories of typical cluster galaxies, when/how they shut down star formation, and how these differ from those of field galaxies are not well understood. The biggest challenge has been to identify the sites of massive cluster progenitors (or ‘protoclusters’) at the peak epoch of their formation (z>~3) as such efforts require sampling of very large volumes and adequate imaging/spectroscopy sensitivities to unambiguously determine protocluster membership. Based on the existing data on the recently discovered massive protocluster (total mass >~10^15 Msun, similar to that of the Coma), I will present several observational evidence that suggest that a systematic search for massive protoclusters can be conducted efficiently utilizing several characteristics that mark the protocluster sites and their constituents. Preliminary results on the properties of several confirmed protoclusters, compared to the field, will be discussed.