Discovered in 1860, cesium was discovered by using the spectrophotometer to look at the lines identifying elements in Durkheim mineral water. Spectral lines for sodium, potassium, lithium, calcium, and strontium were observed. When these metals were chemically removed, the absence of their lines from the spectrum was observed. As the lines began to disappear, two blue lines were unexpectedly observed. The discovery of new spectral lines suggested a new element was to be discovered. Its name was taken from the Latin word coesius meaning "sky blue".

Cesium is an alkali metal. In its solid form, cesium is the softest of all metals. It is silvery white, ductile and easily melts at 28.4^oC. It often exists as a liquid that looks much like mercury. In fact, mercury is the only metal that has a melting point lower than cesium.

The main source of cesium is found in the mineral pollucite, CsAlSi₂O₆. Cesium ores usually contain rubidium, which makes refining cesium metal difficult as rubidium has many critical properties that are nearly identical to cesium. As a result, rubidium and cesium can be difficult to separate from one another. Nowadays, the separation is accomplished by reducing the metals with elemental sodium. In this process, the ores are finely ground and heated to about 650^oC with sodium metal. The result is an alloy of sodium, cesium and rubidium. The three metals are then separated by fractional distillation, much the same way the atmospheric gasses are separated from liquid air (see neon).

Because cesium reacts readily with most kinds of gases, it is used as a getter (i.e. an absorber of unwanted gases) in electronic vacuum tubes and cathode-ray tubes. It is also used in the production of photoelectric devices and atomic clocks. The cesium atomic clocks are so accurate that they vary no more than five seconds in ten generations.