Optical frequency standards are important in the provision of the SI units of length and time.

In 1983, when the present definition of the metre was adopted by the 17^th General Conference on Weights and Measures, the International Committee for Weights and Measures (CIPM) drew up recommendations for its practical realisation, usually referred to as the 'mise en pratique' of the definition of the metre. This included a list of radiations whose stated wavelength in vacuum or stated frequency could be used with the uncertainty quoted, provided that the given specifications and accepted good practice were followed. This list of recommended radiations is periodically updated to take account of new measurements and improvements in techniques for laser stabilisation.

The second is currently defined in terms of a microwave transition in caesium. However optical frequency standards offer the prospect of stabilities and reproducibilities that surpass those of the caesium primary standard, and can be compared to this standard with high accuracy. This points to the potential for an optical re-definition of the second in the longer term. In the meantime, certain optical frequency standards may already be used as secondary representations of the second.