This chapter will discuss the reaction of 1,3-dienes with alkenes to give cyclohexene derivatives in a pericyclic reaction known as a cycloaddition. Conjugated dienes react with alkenes to give cyclohexene products. If methoxyethene is heated with buta-1,3-diene, even higher temperatures and pressures are required. The stereochemistry of the alkene is retained in the cyclohexene product. When there is one substituent on the diene and one substituent on the alkene there is the possibility of diastereomers in the cyclohexene product. Another important aspect of the Diels-Alder reaction can be examined in the reaction of the monosubstituted diene 1-methoxybuta-1,3-diene and the monosubstituted alkene acrylonitrile.