In chemistry, a substitution reaction happens when an atom or a group of atoms is replaced by a different one. Substitution reactions can be of many different types.
In organic chemistry, the main ones are nucleophilic substitution and electrophilic substitution. These take their names from electrophiles and nucleophiles. However, substitution reactions can also happen under other conditions. For example, photochemical reactions (reactions that use light instead of heat) are used to put halogens in a molecule. Another example is using free radicals. Choosing the correct solvent and temperature conditions is very important to make sure that the reaction happens. Sometimes an elimination reaction can happen instead.
The product of a substitution reaction is often called a substituted molecule. For example, phenol can be called a substituted benzene.