A central carbon–carbon bond-forming transformation in organic synthesis, the aldol reaction joins two carbonyl partners by nucleophilic addition of an enol or enolate to an aldehyde or ketone, producing a β-hydroxy carbonyl (an aldol) whose subsequent dehydration affords an α,β-unsaturated carbonyl. Direct catalytic asymmetric aldol reactions generate the nucleophilic donor in situ and use a chiral catalyst to control the facial selectivity of carbonyl addition ^[1]. When the two carbonyls differ, the process is a cross-aldol reaction; enantioselective cross-aldol addition of aldehydes establishes a stereogenic center at the newly formed C–C bond ^[2]. Vinylogous variants extend the same bonding pattern to conjugated enolates, but the defining elementary step remains carbonyl addition rather than conjugate addition to an alkene. Catalytic asymmetric implementations employ metal complexes or organocatalysts. The concept is thereby bounded as a carbonyl–carbonyl coupling, distinct from related enolate alkylations that do not create the aldol oxidation pattern.