This review, presented in three parts, summarizes the results concerning the molecular and electronic structure and the thermodynamic stability of polynitrogen compounds N-n, with n ranging from 4 to 60, as revealed by quantum chemical calculations published in the past 30 years, and where available, by experimental observations. In the present Part 1, attention is paid to the tetranitrogen (N-4) and pentanitrogen (N-5) Systems whose existence and stability have been demonstrated by recent experimental studies. The two following parts will be devoted to the properties of larger clusters, N-n with n greater than or equal to 6. The potential use of the nitrogen clusters as efficient, safe and environment-friendly high energy high-density materials (HEDM) will be discussed. The interactions between these polynitrogen compounds with metals, metal cations and metal complexes, and the factors influencing the ligand stability, will also be examined. (C) 2003 Elsevier B.V. All rights reserved.