The wide applicability of 1-azirines as reagents for the syntheses of many well-known and novel heterocyclic systems is discussed in this article. Although most work has been concentrated on thermal and photochemical cycloaddition reactions of 1-azirines, reactions which proceed via C-N-, N =C-, or C-C-bond cleavage are included. Also, reactions which proceed formally via cycloaddition but give rise to open-chain products are also described. 1. Carbenoid Additions 2. 1,3-Dipolar Reactions 3. Diels-Alder Reactions 4. Ketenes and Ketenimine Additions 5. Isocyanate Additions 6. Miscellaneous Additions 7. Photochemical Cycloadditions