Soon after the commercialization of the Ziegler-Natta catalyst for polyolefin production, efforts were begun to improve the productivity of the catalyst. It was realized that the α- and σ-forms of TiC13have layered crystal structures having only chlorine atoms in the basal planes, Exposed and coordinatively unsaturated Ti sites are only found along c-axis edges of the crystallites. Therefore, if a suitable support is found and Ti atoms can be anchored to its surface with stereoelectronic characteristics resembling those in the AA-TiCl3, then a high activity catalyst would result. This search progressed from metal oxides to metal hydroxides, to metal hydroxychlorides, and finally to metal chlorides, in particular magnesium chloride. The historical developments have been discussed in detail by Karol and Hsieh at this conference.