The chapter discusses the use of N-heterocyclic carbenes (NHCs) in organocatalytic reactions, focusing on their synthesis, properties, and applications. NHCs, particularly triazolylidene carbenes, have become a crucial component in organocatalysis due to their ability to catalyze various transformations. The text highlights the evolution of NHCs from thiazolylidene and imidazolylidene carbenes to triazolylidene carbenes, with the latter being more acidic and thus more suitable for catalytic applications. The synthesis of NHCs is discussed, including methods for preparing thiazolylidene, imidazolylidene, and triazolylidene carbenes. The pKa values of these carbenes are also examined, showing that imidazolylidene carbenes are the least acidic, while thiazolylidene and triazolylidene carbenes are more acidic. The Lewis basicity and nucleophilicity of NHCs are explored, with triazolylidene carbenes being the most nucleophilic. The chapter delves into the umpolung acyl-anion catalysis, where NHCs catalyze the benzoin and Stetter reactions. The benzoin reaction, discovered by Wöhler and Liebig in 1832, has been extensively studied using NHCs, leading to the development of enantioselective catalysts. The Stetter reaction, initially studied by Stetter in the 1970s, has also seen significant advancements, with the development of enantioselective catalysts and the expansion of the reaction scope to include various Michael acceptors and substrates. The text also covers the hydroacylation of double and triple bonds, where NHCs can catalyze the formation of β-functionalized carbonyl compounds from α,β-unsaturated aldehydes. This reactivity is particularly useful for the synthesis of complex molecules. Finally, the chapter discusses extended Breslow intermediates, where NHCs can catalyze umpolung reactions that generate β-functionalized carbonyl compounds from α,β-unsaturated aldehydes. This includes annulation reactions, where NHCs can catalyze the formation of γ-lactones from enals and aryl aldehydes. The text concludes with examples of enantioselective homoenolate additions to acyl phosphonates and 1,2-dicarbonyl compounds, highlighting the versatility and efficiency of NHC-catalyzed reactions.The chapter discusses the use of N-heterocyclic carbenes (NHCs) in organocatalytic reactions, focusing on their synthesis, properties, and applications. NHCs, particularly triazolylidene carbenes, have become a crucial component in organocatalysis due to their ability to catalyze various transformations. The text highlights the evolution of NHCs from thiazolylidene and imidazolylidene carbenes to triazolylidene carbenes, with the latter being more acidic and thus more suitable for catalytic applications. The synthesis of NHCs is discussed, including methods for preparing thiazolylidene, imidazolylidene, and triazolylidene carbenes. The pKa values of these carbenes are also examined, showing that imidazolylidene carbenes are the least acidic, while thiazolylidene and triazolylidene carbenes are more acidic. The Lewis basicity and nucleophilicity of NHCs are explored, with triazolylidene carbenes being the most nucleophilic. The chapter delves into the umpolung acyl-anion catalysis, where NHCs catalyze the benzoin and Stetter reactions. The benzoin reaction, discovered by Wöhler and Liebig in 1832, has been extensively studied using NHCs, leading to the development of enantioselective catalysts. The Stetter reaction, initially studied by Stetter in the 1970s, has also seen significant advancements, with the development of enantioselective catalysts and the expansion of the reaction scope to include various Michael acceptors and substrates. The text also covers the hydroacylation of double and triple bonds, where NHCs can catalyze the formation of β-functionalized carbonyl compounds from α,β-unsaturated aldehydes. This reactivity is particularly useful for the synthesis of complex molecules. Finally, the chapter discusses extended Breslow intermediates, where NHCs can catalyze umpolung reactions that generate β-functionalized carbonyl compounds from α,β-unsaturated aldehydes. This includes annulation reactions, where NHCs can catalyze the formation of γ-lactones from enals and aryl aldehydes. The text concludes with examples of enantioselective homoenolate additions to acyl phosphonates and 1,2-dicarbonyl compounds, highlighting the versatility and efficiency of NHC-catalyzed reactions.