The sulfur dyes represent a huge dye-class and were the last great invention in the field of the early synthetic dyes before the First World War. In two editions of the book Farbstofftabellen by Gustav Schultz, published in 1914 and again in 1931, respectively 52 and 76 different sulfur dyes are listed, with hundreds of trade names and protection by almost 1,000 international patents filed by many dye factories. This clearly shows their importance. The sulfur dyes were first produced by heating organic materials, such as sawdust, bran, straw and other lignin-containing matter, with sulfur and sodium sulfide. Later on, aromatic or heterocyclic products, from the coal tar industry, for example, were used as the source and heated either with sulfur or with sulfur-containing compounds. The sulfur dyes usually consist of complex mixtures; as a result, their chemical composition is not well-defined and is poorly understood. Yet these dyes were widely used for dyeing cotton and played an important role, particularly in black dyeing. They are produced in all colours, but the black, brown, blue and green dyes are of the greatest importance, hence this chapter will mainly focus on these colours.