Construction of a carbon–carbon double bond is the defining operation of alkene synthesis in organic chemistry. The new C=C unit may arise by elimination (dehydrohalogenation, dehydration, or dehydrodecarboxylation), by olefination of a carbonyl or imine, or by stereodefined coupling of two carbon fragments. Photocatalytic dehydrodecarboxylation of carboxylic acids illustrates an elimination route in which loss of CO2 and a hydrogen equivalent produces the alkene ^[1]. Transition-metal-mediated sequences such as alkyne hydroboration followed by tandem Negishi–Suzuki coupling or organoborate migratory insertion furnish tri- and tetrasubstituted alkenes with high regio- and stereochemical definition ^[2]. Carbonyl-equivalent olefinations, including reactions of activated imines with nonstabilized phosphonium ylides, generate the double bond by formal C=X to C=C conversion with tunable alkene geometry ^[3]. Across these manifolds the conceptual core is formation of the olefinic π bond itself, not subsequent functionalization of an already present alkene. Regio- and stereoselectivity are intrinsic attributes of the bonding event, because substitution pattern and E/Z (or facial) outcome are fixed when the double bond is created.