Organic Reaction Mechanisms 2012, the 48th annual volume in this highly successful and unique series, surveys research on organic reaction mechanisms described in the available literature dated 2012. The following classes of organic reaction mechanisms are comprehensively reviewed: Reaction of Aldehydes and Ketones and their DerivativesReactions of Carboxylic, Phosphoric, and Sulfonic Acids and their DerivativesOxidation and ReductionCarbenes and NitrenesNucleophilic Aromatic SubstitutionElectrophilic Aromatic SubstitutionCarbocationsNucleophilic Aliphatic SubstitutionCarbanions and Electrophilic Aliphatic SubstitutionElimination ReactionsPolar Addition ReactionsCycloaddition ReactionsMolecular Rearrangements An experienced team of authors compiled these reviews, ensuring the quality of selection and presentation.