Vat dye is one of the most significant textile dye which was invented in 20th century. Vat dyes are water insoluble colored compound and it can not be use to color cotton or other fibres without modification. Vat dyes are processed to reducing agent to convert into leuco compound which is soluble in water with presence of alkali. In ancient time a kind of container vessel named Vat was used for the reduction of vat dyes, for this vat dyes is so called. Once obtained from plant original vat dyes is indigo but now produced synthetically. Vat dyes has high color fastness which is not common in other dye classes.

Vat dyes are widely used for cellulose fibre such as cotton fibre. Its very widely used for dyeing cotton yarn which is used for making denim or jeans product. 

Vat dyes requires a reducing agent to solubilize to water. Most common reducing agent for vat dyes is sodium dithionite which convert the dye to leuco form to solubilize in water. When its attached to the fabric then leuco dye oxydized to the insoluble state which is intensely colored. Control are often necessary for chemical reaction such as oxidation, reaction, pH. For vat dyeing water need to put in dye bath with temperature of 50º- 60 ºC. Temperature may vary for different dyestuff. Then it may need to add required amount of caustic soda, sodium hydro sulphate with keeping in required temperature. The dyeing process will be in oxygen free mode. Indigo is an example of vat dye class which change color from yellow to green and then blue when hit the air.