Two German chemists, Otto Diels and Kurt Alder, in 1928 reported an organic reaction that would eventually bear their names and win them the Nobel Prize for Chemistry. They had reacted a conjugated diene with a simple alkene to generate a cycloalkane. Their study of the chemistry of this reaction lead to significant advances in the understanding of organic reactions. It provided a useful synthetic route to a wide variety of cyclic organic compounds and, in particular, natural products such as the terpenoids.

The Diels-Alder reaction involves the interaction of a dienophile (literally, a "diene loving" compound) with a diene, under certain conditions. The simplest example of the reaction, the combination of butadiene with ethylene, is actually a very slow reaction requiring a temperature of 392°F (200°C) and pressure to react. However, the addition of an electron withdrawing group to the dienophile (i.e., a conjugated carbonyl) and electron releasing groups to the diene (i.e., a methyl group) results in significant improvement in the rate of reaction. For example, 2,3-dimethyl-1,3-butadiene and 3-buten-2-one react readily at room temperature and pressure.

The utility of the Diels-Alder reaction is extensive but it does have limits. The conjugated diene must be able to adopt a cis geometry with respect to the single bond. Bulky substituents can interfere with the reaction yield. But, because the mechanism for the reaction is pericyclic, there is retention of geometry such that quite specific products can be generated. A cis dienophile will generate a cis product and a trans will generate a trans.

Overall, the Diels-Alder reaction is one of the more useful "named" reactions in organic chemistry and certainly one of the most studied.