The organic heterocyclic compounds have significantly increased in recent years. It is estimated that they make up more than half of the approximately 12 million organic compounds that have been synthesized so far. Their large number necessitates a specific familiarity with them, and for this reason, a separate course is essential, which will mainly develop general principles and examine only parent rings.

In this respect, this book differs from its older namesake (N. Alexandrou, A. Varvogli, and D. Nikolaidis, 1979 edition), which placed considerable emphasis on the spectroscopic properties and chemical behavior of specific compounds of particular interest. Many of these have indeed not lost their importance. Nevertheless, the newer achievements that needed to be mentioned, combined with the realistic approach to the limited scope, necessitated the writing of this new book.

Freed from the "obligation" to present several descriptive elements, as well as some natural products for which there is now a specific course, we have the opportunity to introduce new concepts and elaborate on already known ones in more detail.

By way of example, we present the main points from the new material, which include: the synthetic possibilities of small rings, the treatise on aromaticity, stereoelectronic phenomena, and the development of cyclopropyl additions. There is also an opportunity to become acquainted with phenomena and reactions that, while not exclusive to heterocyclic compounds, have become essential in the chemist's toolkit. These include cine-substitution, ambidentate ions, thiophilic addition, palladium compounds, molecular oxygen, etc.