Limits to time variation of fundamental constants from comparisons of atomic frequency standards

Comparisons between frequency standards based on optical and microwave transitionsin atoms and ions currently provide the most stringent laboratory limitson present-day local variation of the fundamental constants.This paper discusses the sensitivity of these frequency standards to variation ofthe electromagnetic and strong interactions, summarises the constraints onvariation of these interactions which can be obtained from currently-available data, and discusses the magnitude of improvements in these limits which is anticipated fromexperiments currently underway.