Polymerization process in a slurry loop reactor having a slurry loop reaction zone with a volume of at least 50 m3 and an internal diameter (D) of 50 cm or greater, a feed inlet for monomers and diluent, a catalyst inlet for polymerisation catalyst, and a discharge conduit for removal of polymer. Monomer, diluent and catalyst are passed into the reaction zone through their respective inlets where they form a slurry of polymer solids having a polymer solids concentration in the reaction zone of greater than 20 wt %. The space-time yield (STY) is greater than 100 kg/h/m3, and the catalyst inlet is an inlet pipe such that no part of the inlet pipe protrudes beyond the wall of the reaction zone and into the reaction zone by more than 1/10th of the diameter of the reaction zone at the point where the inlet pipe joins the reaction zone.