Chromium-based catalysts are the most important ethylene polymerization and oligomerization catalysts widely applied for industrial production of polyethylene and 1-hexene. Phillips chromium catalyst is a well-known heterogeneous catalyst for commercial production of HDPE products, which accounts for more than 40% of world production annually. The Chevron-Phillips Cr-based homogeneous catalyst system is the first commercialized catalyst for the production of 1-hexene through selective ethylene oligomerization. Although a great success with these Cr-based catalysts has been achieved in industrial applications, there are still many debates in the academic field concerning the precise structure of active chromium species, the oxidation states of chromium center, the effects of cocatalysts/ligands and the catalytic mechanisms. During the last decades, a step-forward mechanistic understanding has been achieved through extensive and successive investigations on these Cr-based catalysts for ethylene polymerization/oligomerization. In addition, the progress in mechanistic understanding on alkyne cyclotrimerization by the same Phillips catalyst and ethylene polymerization over Mo-based catalyst are also covered. The later might be served as an alternative green catalyst for the industrial production of polyethylene.