The Diels-Alder reactions of the cycloalkenes, cyclohexene through cyclopropene, with a series of dienes-1,3-dimethoxybutadiene. cyclopentadiene, 3,6-dimethyltetrazine, and 3,6-bis(trifluoromethyl)-tetrazine-were studied with quantum mechanical calculations and compared with experimental values when available. The reactivities of cycloalkenes as dienophiles were found by a distortion/interaction analysis to be distortion controlled. The energies required for c-ycloalkenes to be distorted into the Diels-Alder transition states increase as the ring size of cycloalkenes increases from c-yclopropene to cyclohexene, resulting in an increase in activation barriers. The reactivities of the dienes are controlled by both distortion and interaction energies. In normal Diels-Alder reactions with cycloalkenes, the electron-rich 1,3-dimethoxybutadiene exhibits stronger interaction energies than cyclopentadiene, but the high distortion energies required for 1,3-dimethoxybutadiene to achieve transition-state geometries overtake the favorable interaction, resulting in higher activation barriers. In inverse-electron-demand Diels-Alder reactions of 3,6-dimethyltetrazine and 3,6-bis(trifluoromethyl)tetrazine, the reactivities are mainly controlled by interaction energies.