Direct dyes are a class of dyes capable of dyeing cellulosic fibers directly—without the need for a mordant—in neutral or weakly alkaline media under boiling conditions. They bind to fibers via hydrogen bonding and van der Waals forces and are widely used in industries such as cotton textiles, silk, leather, papermaking, knitting, ribbons/tapes, and wool/linen processing. Based on chemical structure, they can be classified into types such as azo and stilbene; based on application properties, they are categorized into ordinary direct dyes, direct light-fast dyes, and direct azo dyes.