A comprehensive approach to heterocyclic chemistry, based on the concept of PI [ ] -excessive and PI [ ] -deficient ring systems. Introduces the categorical similarities and differences among the varied heterocyclic systems. Explains the logical consistency of the numerous systhesis and relative reactivities of heterocyclic compounds, as demonstrated not only by their chemical reactions, but by their spectroscopic properties as well. Also provides highly useful tabulation of nuclear magnetic resonance data for proton, carbon and nitrogen, mass spectral fragmentation, and the results of unified molecular orbital calculations.