The main stages of the physical preparation of the plant, <i>Indigofera tinctoria</i>, by pulping, drying, powdering, and fermenting are first reviewed; the text then indicates the chemical processesmhydrolysis, oxidation, and reoxidationmby which a durable indigo dyeing was achieved. Mention is made of the fact that liquid phase chromatography can be used to distinguish between indigo and pastel blues, a useful process in dating and provenancing old textiles. (See also <i>AATA</i> <b>27-749</b>.)