We describe and compare the proposed mechanisms of ethene polymerization by the metallocene/methylaluminoxane (MAO) catalyst in terms of quantum chemical calculations. In combination with the Cp2ZrMe2 precatalyst, we employ two models for MAO, produced by hydrolysis of trimethylaluminum (TMA). Both MAOs contain associated TMA as a key ingredient for cocatalytic activity. The TMA association/dissociation equilibrium in the MAOs controls the mechanism of catalyst activation and suggests preference for catalyst activation via [AlMe2]+ abstraction from the MAO by the precatalyst rather than via Lewis-acidic abstraction of the leaving group from the precatalyst by the MAO. Solvent interactions increase the relative concentration of Lewis-acidic sites. Chlorination of MAO facilitates the catalytic processes. Studies as a function of precatalyst structure reproduce the general experimental observations of the easier catalyst activation by zirconocenes than by hafnocenes and the positive effects of adding a dimethylsilyl bridge and replacing the cyclopentadienyl with an indenyl ligand. This study provides a starting point for rational control of the behavior of the metallocene/MAO catalyst system.