An addition reaction is a reaction in which atoms or groups are added across a multiple bond, increasing the number of groups attached to carbon and reducing the unsaturation. It matters as the characteristic reaction of compounds with double or triple bonds, such as alkenes and carbonyls. It is the opposite of an elimination reaction, in which groups are removed to form a multiple bond, and differs from substitution, in which one group merely replaces another.

加成反应是原子或基团加到多重键两端的反应,使连接在碳上的基团数目增加、不饱和度降低。它之所以重要,是因为它是含双键或三键化合物(如烯烃和羰基)的特征反应。它与消除反应相反(消除是脱去基团以形成多重键),也不同于取代反应(取代只是一个基团替换另一个)。