Apparatus for olefin polymerization comprising: an olefin containing feedstock distribution assembly including a feedstock inlet and a plurality of feedstock outlets (15a, 15b), said distribution assembly being adapted and arranged to receive feedstock via said feedstock inlet, divide said feedstock into a plurality of separate streams and deliver each of said streams to a respective one of said outlets; a plurality of parallel loop reactors (202a, 202b), each of said reactors comprising structure defining a reaction zone, said structure being adapted and arranged to facilitate the conduct of an exothermic olefin polymerization reaction on an olefin polymerization reaction mixture in said zone, said structure comprising inlet and outlet connections arranged in fluid communication with said zone, and a recirculation system (204a, 204b) including a pump (25a, 25b) arranged and adapted to circulate said reaction mixture in said zone, each said feedstock outlet (15a, 15b) being connected in fluid communication with the inlet connection of a respective reactor, each said recirculation system (204a, 204b) being adapted and arranged to circulate the reaction mixture in its respective zone independently of the introduction of feedstock into said zone via a respective inlet connection; a crude polyolefin product collection assembly including a plurality of crude polyolefin product inlets (55a, 55b) and a crude polyolefin product outlet, each said crude polyolefin product inlet being connected in fluid communication with the outlet connection of a respective reactor; a respective catalyst composition inlet (30a, 30b) in fluid communication with each reaction zone, each said catalyst composition inlet being adapted and arranged to introduce catalyst composition into said recirculating olefin polymerization reaction mixture in said zone, and a respective catalyst modifier inlet (16a, 16b) in fluid communication with each said reaction zone, each said catalyst modifier inlet being adapted and arranged to introduce catalyst modifier into said recirculating olefin polymerization reaction mixture at a rate which is independent of the rate of introduction of the catalyst composition. an olefin containing feedstock distribution assembly including a feedstock inlet and a plurality of feedstock outlets (15a, 15b), said distribution assembly being adapted and arranged to receive feedstock via said feedstock inlet, divide said feedstock into a plurality of separate streams and deliver each of said streams to a respective one of said outlets; a plurality of parallel loop reactors (202a, 202b), each of said reactors comprising structure defining a reaction zone, said structure being adapted and arranged to facilitate the conduct of an exothermic olefin polymerization reaction on an olefin polymerization reaction mixture in said zone, said structure comprising inlet and outlet connections arranged in fluid communication with said zone, and a recirculation system (204a, 204b) including a pump (25a, 25b) arranged and adapted to circulate said reaction mixture in said zone, each said feedstock outlet (15a, 15b) being connected in fluid communication with the inlet connection of a respective reactor, each said recirculation system (204a, 204b) being adapted and arranged to circulate the reaction mixture in its respective zone independently of the introduction of feedstock into said zone via a respective inlet connection; a crude polyolefin product collection assembly including a plurality of crude polyolefin product inlets (55a, 55b) and a crude polyolefin product outlet, each said crude polyolefin product inlet being connected in fluid communication with the outlet connection of a respective reactor;