A method for identifying a catalyst composition for use in the heterogeneous Ziegler-Natta addition polymerization of an olefin monomer, said catalyst composition comprising a procatalyst comprising a magnesium and titanium containing procatalyst and a cocatalyst said method comprising: a) providing a library comprising at least one procatalyst compound, b) forming a catalyst composition library by contacting the member of said procatalyst library with one or more cocatalysts and contacting the resulting mixture with an olefin monomer under olefin polymerization conditions thereby causing the polymerization reaction to take place, c) measuring at least one variable of interest during the polymerization, and d) selecting the catalyst composition of interest by reference to said measured variable.