Catalysts that have been preactivated and/or prepolymerized are disclosed whereby a magnesium and titanium-containing procatalyst component is contacted with a co-catalyst and an external electron donor (and optionally with an olefin monomer to prepare a prepolymerized catalyst) prior to polymerization to form a preactivated and/or prepolymerized catalyst. The preactivated and/or prepolymerized catalyst then is separated from the mixture, and dried to form a solid preactivated and/or prepolymerized catalyst. This dried catalyst then can be stored and subsequently shipped to a polymerization site where it can be used in gas phase polymerization. The preactivated and/or prepolymerized catalyst can be used in gas phase polymerization as extremely high activity catalysts, and do not cause a rapid rise in reaction temperature causing overheating, undesirable formation of agglomerates, coagulation of polymer, and ultimately, reactor failure.