A review deals with the structure, synthesis, and reactivity of compounds containing pentamethine systems (i.e. pentamethinium salts, aminopentadienals, glutaconoic aldehydes, and cyanine dyes). The syntheses of these compounds via ring-opening of the heterocycles, by building the skeleton, or by addition reactions are described, as well as their reactivity to nucleophilic and electrophilic reagents, including the ring-closure reactions.