The latest member of the azole family, the pentazolate or cyclo-N5−, has received increased attention since its first mass-spectral detection by Christe et al. in 2002. As it is carbon- and hydrogen-free, the pentazolate anion can release large amounts of energy while simultaneously decomposing to environmentally friendly nitrogen gas. Due to these attractive qualities, cyclo-N5− and related compounds are essential in the advancement of high-energy-density materials (HEDMs) research. This review aims to provide a consolidated report on all research done on cyclo-N5−, with a focus on pentazoles as energetic materials and on their experimental synthesis. Included in this review are the following: ① the historical significance of cyclo-N5−; ② precursors of cyclo-N5−; ③ synthesis routes of cyclo-N5− with a focus on arylpentazole precursors; ④ factors affecting the stability of cyclo-N5−; ⑤ energetic performances of current energetic cyclo-N5−-containing compounds; and ⑥ future possible experimental research. This review is a comprehensive summary of the current understanding of cyclo-N5−, in an effort to further understand the potential of this anion for adoption as a powerful and environmentally friendly next-generation explosive.