The large-scale production of polyolefins has been enhanced by the development of very high activity MgCl2-supported Ziegler-Natta catalysts. Higher activities mean lower production costs or, as in the cases of the UNIPOL or SHERIPOL gas-phase processes, new production technologies. However, despite their abundant use by industry and an intensive research effort by academic and industrical laboratories, a complete understanding of the catalyst's behavior is lacking. Not only is the catalyst a complex mixture of components (such as highly reactive aluminum alkyls) possessing multiple site types, but physicochemical phenomena such as mass transfer limitations across the gas/liquid interface in slurry reactors [58], catalyst fragmentation behavior at the start of polymerization [155], monomer transport to the active sites and heat transport away [61–63], and possible monomer sorption equilibria in the amorphous polymer regions [74] can further complicate the observed kinetics. The objective of this paper is to review the recent advances in the field in terms of its structure, mechanism, and kinetics of the catalyst based on the accumulation of results from many laboratories. The first part of the review will examine the make-up of a supported catalyst by breaking it down and analyzing its components. From this a discussion of the active site structure and mechanism will evolve. Finally, the polymerization rate kinetics will be reviewed. There is a vast amount of literature for propylene polymerizations and a dearth of publications about these catalysts with respect to their ethylene polymerization kinetics, so much of the discussion will be in reference to propylene polymerization.