Acid Red 87, universally known as Eosin Y, is a historically significant xanthene dye. It is celebrated for its characteristic bright, yellowish-pink colour and its strong red fluorescence in solution. Its primary and most famous application is in biology and histology as a counterstain, where it colours cytoplasm and other cell structures pink. Industrially, it is used as a specialty colourant for inks and for dyeing paper and certain textiles where its unique fluorescent shade is desired.