A substitution reaction is a reaction in which one atom or group attached to a carbon is replaced by another atom or group. It matters because substitution is one of the three basic ways an organic molecule can be transformed and is the route to many functional-group conversions. It is to be contrasted with an addition reaction, in which groups are added across a multiple bond, and with an elimination reaction, in which groups are removed to create a multiple bond.

取代反应是碳上连接的一个原子或基团被另一个原子或基团替换的反应。它之所以重要,是因为取代是有机分子三种基本转化方式之一,也是许多官能团转化的途径。它要与加成反应(基团加到多重键两端)和消除反应(脱去基团以生成多重键)相区别。