This treatise pertains to dyes composed of a central polymethine moiety and two cyclic terminal subunits. The polymethine linker can be unsubstituted or substituted, and at least one terminal subunit is a heterocyclic system. The classes of compounds reviewed range from classical cyanines, ?rst s- thesized in the 1850s, to hemicyanines, to styryl dyes, to merocyanines, to coumarinpolymethines, andtosquarylium dyesthatweresynthesized forthe ?rst time in the 1960s. These structurally diverse classes of compounds have one commondenominator, namely electronconjugation that involves the t-...