Process for the gas phase polymerization of olefins, in particular, with recycle of fines to the reaction zone. The process includes (a) polymerizing an olefin in a reaction zone in which a bed of polymer particles is maintained in an agitated state by passing a fluid stream through the bed, (b) withdrawing the fluid stream from the reactor after it has passed through the bed of polymer particles, (c) separating entrained solids from the withdrawn fluid stream, and (d) returning the separated entrained solids to the reaction zone. The polymer production rate of the process is at least 40 tonnes/hour, and the separated solids from step (c) are returned to the lower half of the bed of polymer particles in the reaction zone.