This volume is the first of a series of volumes on cyclic phosphorus-nitrogen compounds and includes monocyclic three-, four-, and five-membered ring species containing skeletons built exclusively of phosphorus and nitrogen atoms. The chemistry and physics of many types of these differently substituted rings are so closely related that it was possible to comprehensively summarize those compounds that have the same ring structure. By far the largest part of this volume covers the P2N2 ring species, in particular the numerous examples containing the completely saturated #65533;PN#65533;2 skeleton.