Process for polymerizing in a loop reactor an olefin monomer optionally together with an olefin comonomer in the presence of a polymerization catalyst in a diluent to produce a slurry including solid particulate olefin polymer and the diluent. The Froude number is in the range 3 to 10, the internal diameter of the loop reactor is over 600 millimeters, the solids concentration of the slurry in the loop reactor is above 20% by volume and, when the polymer produced is polyethylene and the diluent is an alkane, the solids concentration is above 40 wt % based on the total weight of the slurry.