Three possible mechanisms of ethylene polymerization in the presence of zirconocene catalysts have been described through density functional calculations. About 20 structures (reactants, intermediates, transition states, and products) have been located on the potential energy surface. Only two mechanisms, called frontside and stepwise backside, appear to be competitive. The frontside mechanism has already been described in the literature, but we present here some important new aspects. in particular, we discuss the important role of the perpendicular form of the olefin Jr-complex for the frontside process. The stepwise backside mechanism is reported for the first time. The energy barriers in both mechanisms are on the order of 3 kcal/mol, which is compatible with the experimental and theoretical values available for similar processes. The energy difference between both mechanisms is certainly below the computational method accuracy, and a definitive conclusion regarding the most favorable mechanism cannot be drawn. The influence of other factors such as substituents on the catalysts or the size of the polymer chain could be fundamental.