Publisher Summary This chapter presents the knowledge concerning the molecular and electronic structure of pyrrole. It also surveys the spectroscopic and nonspectroscopic physical properties of pyrrole and its simple derivatives. Because of the instability or nonavailability of many compounds, the study of pyrroles has lagged behind that of other heterocyclic systems. The chapter discusses the structure of pyrrole based on molecular and electronic structure and aromaticity; and molecular association. The physicochemical properties such as nonspectroscopic measurements and spectroscopic measurements are also presented. Deductions drawn from early data can be suspected because of the doubtful purity of the compounds and majority of more reliable results come from investigations of more stable pyrroles having electron-withdrawing substituents. Recent improvements in synthetic and instrumental methods have enabled more accurate data to be accumulated. Considerable use has been made of infrared (IR) measurements, but interest has been moving recently to the newer techniques of nuclear magnetic resonance (NMR) and mass spectrometry. Isolated investigations using electron spin resonance (ESR)7-9 and magnetic susceptibility10-17 techniques have also been made on pyrrole-metal complexes and pyrrolyl radicals.