The mechanism of the enantioselective, hydrazide-catalyzed Diels-Alder cycloaddition was investigated in detail. Both the formation of the reactive iminium species and the hydrolysis of the product iminium intermediates were found to be extremely rapid, leaving the cycloaddition as the kinetically significant step. Mechanistic studies using NMR showed that a retro-Diels-Alder reaction occurred during the catalytic cycle, suggesting a thermodynamic component to the reaction.