John L. Hall 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: A historical account of mechanical clockworks is followed by an in-depth survey of methods to measure optical frequencies with very high resolution. The evolution of the experimental programme at JILA, University of Colorado, is described in detail. The re-definition of metre is discussed.