Substitution reactions involve the replacement of an atom or group of atoms in a molecule with another atom or group of atoms. Examples of substitution reactions include nucleophilic substitution and electrophilic substitution. In a nucleophilic substitution reaction, a nucleophile (an electron-rich species) attacks the substrate (the molecule undergoing the reaction) and replaces one of its atoms or groups of atoms. In an electrophilic substitution reaction, an electrophile (an electron-deficient species) attacks the substrate and replaces one of its atoms or groups of atoms.