Vicinal difunctionalization reactions play an important role in modern synthetic organic chemistry. They provide access to complex structures in a stereocontrolled fashion and act as powerful, attractive, convergent elements in synthetic strategy. Consequently, examples of these reactions are numerous. Among them may be cited the Diels–Alder reaction, which results in vicinal dialkylation of a dienophile; epoxidation–functionalization of alkenes which results in 2-substituted alkanols carbenoid additions to alkenes resulting in cyclopropanes; organometalation–functionalization of alkynes giving vicinally disubstituted alkenes, and the additions of halides and acyl halides to alkenes using Friedel–Crafts catalysts. [2 + 2] Photocycloadditions and 1,3-dipolar cycloadditions are but two of many more examples. New reactions are introduced regularly, such as radical cyclization–trapping, which recently has been applied to a synthesis of prostaglandin F2α.