Recent developments in the area of polymerization reactor design and optimization have been highlighted using polyethylene terephthalate (PET) as an example. Both the DMT and the TPA routes for its manufacture have been discussed and it has been demonstrated that a good understanding of the various physical processes present in industrial reactors is required before good models can be developed. The simulations carried out have been tested on some industrial scale reactors as well as pilot plants and improvements have been suggested based on these studies.