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A Claisen condensation begins when base removes an α-hydrogen from an ester to form an enolate. The enolate attacks a second ester carbonyl, and collapse of the tetrahedral intermediate expels an alkoxide, forming a β-keto ester. A subsequent deprotonation helps drive the reaction; acidic work-up then reprotonates the product. Reaction design matters: an alkoxide matching the ester’s OR group helps avoid trans-esterification, and at least one ester partner needs an α-hydrogen. The lesson also introduces crossed Claisen reactions, the intramolecular Dieckmann variant, and common limitations. Finally, it connects β-keto esters to further synthetic transformations, including alkylation and decarboxylation.