A number of proposed models of the active sites in TiCl4/MgCl2 heterogeneous Ziegler-Natta catalysts are examined using density functional methods. Using a number of different models for the surface, the sites formed when unreduced TiCl4 is adsorbed onto MgCl2 are predicted to be unstable. In contrast, TiCl3 and TiCl2,are found to bind strongly to MgCl2. Alternative models of the surface where one or all of the Mg atoms is replaced with Ti(II) give larger binding energies for TiCl4. Each step of the ethylene polymerization reaction is considered for each of the site models that are expected to be active. Two possible termination mechanisms, chain transfer to the monomer and beta-hydrogen elimination, are also examined. The sites formed from TiCl4 bind ethylene more weakly than those formed from TiCl3. No particular trends with respect to the type of site are found for the barrier to insertion, but the direction of approach of the ethylene molecule is important. The beta-hydrogen elimination termination mechanism is predicted to be much less important than chain transfer to the monomer. From a comparison of ethylene insertion and termination barriers, the TiCl3-based sites on the TiCl2 surface and the TiCl3-based Corradini site on MgCl2 are shown to be very poor catalysts while the others, in particular the TiCl3-based edge site on MgCl2, appear to be moire promising models of the actual active sites.