This chapter focuses on the xanthenes, an important group used as dyes and pigments. This chemical group of synthetic dyes was discovered by Adolf Baeyer in 1871. He discovered Fluorescein, a yellow colourant which had limited use as a textile dye, but is still currently used as a biological stain. However, the invention of Fluorescein was very important because it is a building block for other components of this dye class which can be used as textile dyes, for inks and in some cases as organic pigments. First, the Eosins and their derivatives the Phloxines were developed; the dye-class was then extended some years later by the Erythrosines, with their derivatives the Rose Bengals. All these dyes are acid dyes. At the end of the nineteenth century, the basic Rhodamines and their derivatives were invented.