Orbital symmetry plays a crucial role in Diels-Alder cycloadditions by dictating the interaction between the Highest Occupied Molecular Orbital (HOMO) of the diene and the Lowest Unoccupied Molecular Orbital (LUMO) of the dienophile. This pericyclic reaction requires that the orbitals overlap in a constructive manner, which is only possible when the symmetries of the interacting orbitals are compatible. The HOMO of the diene must have a p-orbital symmetry that allows "side-on" overlap with the p-orbital symmetry of the LUMO of the dienophile. This symmetry-matching sets the stage for a concerted mechanism that leads to the formation of a new six-membered ring with conservation of orbital symmetry throughout the reaction.