It is a metallic element, discovered by spectroscopy in 1860 by Robert Bunsen and Gustav Kirchhoff. Ductile, soft as wax, and silver-white, it is the most alkaline element Alkali metals and the most reactive metal.

Caesium metal is used in photoelectric cells and various optical instruments; caesium compounds, in glass and ceramic production. The cesium-137 radioactive isotope is used to treat cancer.