Nucleophilic substitution and base-induced elimination are two of the most widely occurring and versatile reactions in organic chemistry, both in the laboratory and in biological pathways. We’ll look at them closely in this chapter to see how they occur, what their characteristics are, and how they can be used. We explore nucleophilic substitution and elimination reactions together because they often compete under similar conditions, with the choice between them hinging on subtle differences in reactants and reaction conditions. Both types of reactions involve the interaction of a molecule with either a nucleophile or a base, where the key lies in understanding how nucleophiles, which seek to form new bonds, differ from bases, which tend to abstract protons. This duality of roles—whether a species acts as a nucleophile or a base—depends on the molecular environment, making it crucial to discuss these reactions side by side to grasp their strategic interplay in organic synthesis.