The Ullmann reaction is a copper-mediated coupling of aryl halides that forms a biaryl carbon–carbon bond. In its classical form, two aryl halide molecules are coupled by copper at elevated temperature, with loss of halide and creation of the bond between their ipso carbons ^[1]. Modern Ullmann-type chemistry extends the copper-mediated or copper-catalyzed manifold to arylation of nitrogen, oxygen, and sulfur nucleophiles, but these C–heteroatom variants are more precisely designated Ullmann-type amination, etherification, or thioetherification ^[2]. Proposed mechanisms depend on ligand, substrate, and conditions and can involve copper(I)/copper(III) cycles, single-electron pathways, or surface processes. The defining boundary of the original Ullmann reaction is reductive aryl–aryl coupling, which distinguishes it from nucleophilic aromatic substitution and from palladium-catalyzed cross-couplings. Both coupling partners are aryl fragments, and copper is the characteristic metal mediator.