Homogeneous nickel catalysts have a considerable track record for mediating ethylene chain growth in the form of oligomerization and more recently polymerization. Within the polymerization arena, high molecular weight materials incorporating various degrees of branching, anywhere from linear to moderately branched through to hyperbranched, highlight the versatility of this type of catalyst. This review focuses on recent progress related to structural modifications made to the pre-catalyst, and in particular to the multidentate N_imine-ligand manifold, and how these changes impact on thermal stability and activity of the catalyst as well as the microstructural properties of the polyethylene and the distribution of the oligomeric fractions. In addition to ongoing process development directed towards commodity-type polyolefinic materials, the emergence of nickel catalysts that can form elastomeric-type materials from a single ethylene feed, without the addition of a high-cost α-olefin such as 1-hexene or 1-octene, offers considerable opportunities for future commercial applications.