The search for precision in time measurement has been a driving force behind innovations in the watch industry since the 17th century. The pendulum, the balance spring and then the quartz make it possible to multiply the precision of clocks. From the Second World War onwards, the precise measurement of time passed from the hands of watchmakers and astronomers to those of physicists. From 1967, the measurement is determined by a microscopic phenomenon: the oscillation of caesium atoms. One femtosecond – 0.000 000 001 seconds is measurable. This precision, hidden to ordinary people, is essential to the organisation of today’s human society: geolocation, navigation, transport, telecommunications, possible thanks to the extreme precision of the time measurement.