Suitably substituted electron-deficient 1,2,3-triazoles exist in equilibrium with their ring-opened α-diazoimine tautomers. This phenomenon was used to facilitate the generation of synthetically useful metallo azavinyl carbene intermediates. In this short review, we describe recent developments in the transition-metal-catalyzed decomposition of electron-deficient (annulated) 1,2,3-triazoles to form metallo azavinyl carbenes, and their application in the synthesis of highly functionalized nitrogen-based heterocycles and building blocks through various synthetic transformations, such as transannulation, cyclopropanations, insertion reactions, ylide formation/rearrangements, and ring expansions. 1 Introduction 2 Ring-Chain Isomerism 3 Synthesis of N-Sulfonyl-1,2,3-triazoles 4 Denitrogenative Transformations of 1,2,3-Triazoles 4.1 Transannulations with Nitriles and Alkynes 4.2 Transannulations with Alkenes or Carbonyl Compounds 4.3 Transannulations with (Hetero)cumulenes or 1,3-Dienes 4.4 Insertions into X-H or C-X bonds 4.5 Rearrangement/Ring Expansion 4.6 Miscellaneous Transformations 5 Summary and Outlook. © Georg Thieme Verlag Stuttgart - New York.