Bismuthinidenes are a special family of organobismuth compounds with a general formula RBi (R = monoanionic ligand) that retain their monomeric structure. The central bismuth atom has +I oxidation state and carries either two lone pairs of electrons in the case of singlet bismuthinidenes or possesses a formally diradical triplet character. Regardless of which group is considered, all bismuthinidenes are inherently electron-deficient and highly reactive compounds that have remained for a long time elusive. Now, there are two main ways to stabilize them. At ambient conditions, stable singlet bismuthinides are accessible by stabilization through adduct formation with a suitable Lewis base(s), while triplet bismuthinidenes have only been isolated very recently using steric protection by extremely bulky organic ligands. This chapter aims to summarize advances in the field of these intriguing low-valent bismuth species, focusing mainly on their synthesis, structure, application in coordination chemistry, chemical bond activation, and catalysis.