Theodor W. Hänsch held his Nobel Lecture December 8, 2005, at Aula Magna, Stockholm University. He was presented by Professor Sune Svanberg, Chairman of the Nobel Committee for Physics.

Summary: Development of the frequency comb technique, which allows very high resolution of optical frequencies, began in the 1970s. A breakthrough came in 1999 with a laser-based optical frequency comb synthesizer, which has a very high uniformity. This allowed the accuracy of clocks to improve markedly; from about one second per day in the year 1800 to about one picosecond per day in 2000. A review is given here of the applications of improved atomic clocks.