Dyeing is the application of dyes or pigments on textile materials such as fibers, yarns, and fabrics with the goal of achieving color with desired color fastness. Dyeing is normally done in a special solution containing dyes and particular chemical material. Dye molecules are fixed to the fiber by absorption, diffusion, or bonding with temperature and time being key controlling factors. The bond between the dye molecule and fiber may be strong or weak, depending on the dye used. Dyeing and printing are different applications; in printing, color is applied to a localized area with desired patterns. In dyeing, it is applied to the entire textile.

The primary source of dye, historically, has been nature, with the dyes being extracted from plants or animals. Since the mid-19th century, however, humans have produced artificial dyes to achieve a broader range of colors and to render the dyes more stable for washing and general use. Different classes of dyes are used for different types of fiber and at different stages of the textile production process, from loose fibers through yarn and cloth to complete garments.

Acrylic fibers are dyed with basic dyes, while nylon and protein fibers such as wool and silk are dyed with acid dyes, and polyester yarn is dyed with dispersed dyes. Cotton is dyed with a range of dye types, including vat dyes, and modern synthetic reactive and direct dyes.

The word 'dye' (/ˈdaɪ/, DIE) comes from the Middle English 'deie', and from the Old English 'dag' and 'dah'. The first known use of the word 'dye' was before the 12th century.

The earliest dyed flax fibers have been found in a prehistoric cave in Georgia and dates back to 34,000 BC. More evidence of textile dyeing dates back to the Neolithic period at the large Neolithic settlement at Çatalhöyük in southern Anatolia, where traces of red dyes, possibly from ocher, an iron oxide pigment derived from clay, were found. In China, dyeing with plants, barks, and insects has been traced back more than 5,000 years. Early evidence of dyeing comes from Sindh province in Ancient India modern day Pakistan, where a piece of cotton dyed with a vegetable dye was recovered from the archaeological site at Mohenjo-daro (3rd millennium BCE). The dye used in this case was madder, which, along with other dyes such as indigo, was introduced to other regions through trade. Natural insect dyes such as Cochineal and kermes and plant-based dyes such as woad, indigo and madder were important elements of the economies of Asia and Europe until the discovery of man-made synthetic dyes in the mid-19th century. The first synthetic dye was William Perkin's mauveine in 1856, derived from coal tar. Alizarin, the red dye present in madder, was the first natural pigment to be duplicated synthetically in 1869, a development which led to the collapse of the market for naturally grown madder. The development of new, strongly colored synthetic dyes followed quickly, and by the 1870s commercial dyeing with natural dyestuffs was disappearing. An important characteristic was light-fastness - resistance to fading when exposed to sunlight using industrial techniques such as those developed by James Morton.

Dyeing can be applied at various stages within the textile manufacturing process; for example, fibers may be dyed before being spun into yarns, and yarns may be dyed before being woven into fabrics. Fabrics and sometimes finished garments themselves may also be dyed. The stage at which a product is dyed varies depending on its intended end use, the cost to the manufacturer, its desired appearance, and the resources available, amongst other reasons. There are specific terms to describe these dyeing methods, such as:

There are various terms used in the manufacturing and marketing industries depending on the method used to dye the substrate. For example, "stock dyed" refers to dyeing the fibers before making the yarn, "yarn dyed" refers to dyeing the yarns before producing fabrics, and "piece dyed" or "fabric dyed" refers to dyeing the yarns after they are converted into fabric. The fastness of fiber- and yarn-dyed materials is superior to that of fabrics.

The primary objective of the dyeing process is to achieve uniform color application in accordance with a predetermined color matching standard or reference on the substrate, which may be a fiber, yarn, or fabric, while meeting specified colour fastness requirements. Tie-dye and printing are the methods where the color is applied in a localized manner.