Organic Reaction Mechanisms 2019, the 55th annual volume in this highly successful and unique series, surveys research on organic reaction mechanisms described in the available literature dated 2019. The following classes of organic reaction mechanisms are comprehensively reviewed: Reaction of Aldehydes and Ketones and their Derivatives Reactions of Carboxylic, Phosphoric, and Sulfonic Acids and their Derivatives Oxidation and Reduction Carbenes and Nitrenes Nucleophilic Aromatic Substitution Electrophilic Aromatic Substitution Carbocations Nucleophilic Aliphatic Substitution Carbanions and Electrophilic Aliphatic Substitution Elimination Reactions Polar Addition Reactions Cycloaddition Reactions Molecular Rearrangements Radicals An experienced team of authors compile these reviews every year, so that the reader can rely on a continuing quality of selection and presentation.