Publisher Summary Most hydroxy-substituted azaheteroaromatic compounds exist predominately in the NH/carbonyl tautomeric form, and these compounds have been widely discussed. This chapter provides an overview of azaheteroaromatic compounds and I-azetin-4-ones—these are those azaheterocycles that contain in the ring at least one carbonyl group and one or two nitrogen atoms in the form of C=N or N=N bonds. Many of these compounds are unstable but their importance as reactive intermediates is established, and many of them could be used as synthons in the preparation of pharmacologically active compounds. The chapter describes the free existence of I-azetin-4-one and the nucleophilic substitution reactions of 4-substituted 2-azetidinone that follows an elimination-addition pathway. 4-Phenyl-l,2,4-triazoline-3,5-dione acts as a dienophile by in situ reaction with butadiene, cyclopentadiene, cycloheptatriene, and bicycloheptadiene. Thus, it is possible to compare the reactivity of the cis-azo dienophile with trans-azo dienophiles.