The naturally occurring precursor of indigo is indican, a colourless, water-soluble substance that is easily hydrolyzed to glucose and indoxyl; the latter is converted to indigo by mild oxidation, such as exposure to air.

The chemical structure of indigo was announced in 1883 by Adolf von Baeyer; a commercially feasible manufacturing process was in use by the late 1890s. The method, still in use throughout the world, consists of a synthesis of indoxyl by fusion of sodium phenylglycinate in a mixture of caustic soda and sodamide.

Indigo can be converted to numerous simpler compounds, but the only chemical reaction of practical importance is its reduction to the soluble yellow leucoindigo, in which form it is applied to textile fibres and reoxidized to indigo.

in botany, any shrub or herb of the genus Indigofera within the pea family (Fabaceae). Most species occur in warm climates and are generally silky or hairy. The leaves are usually divided into smaller leaflets. The small rose, purple, or white flowers are borne in spikes or clusters. The fruit is a pod, usually with a thin partition between the seeds.

Some species, particularly I. sumatrana and I. arrecta, were once an important source of indigo dye. The cultivation of indigo plants and the extraction of the dyestuff were an important industry in India up to the beginning of the 20th century. Synthetic indigo, developed about that time, gradually replaced natural indigo as a dyestuff.