The ability to  cool neutral atoms and trapped ions using laser light has brought a new generation of ultra-stable clocks to existence in the microwave and optical domains. We will introduce the basic concepts, and discuss the main factors contributing to the frequency stability and accuracy of these
frequency standards.

We will then present some applications of these ultra-stable clocks in fundamental physics such as the search for a possible drift of fundamental constants and  tests of general relativity. The possibility to
compare clocks in various part of the electromagnetic spectrum using mode-locked femtosecond lasers will lead to dramatic advances  in the sensitivity of these tests.

Finally, the interest of space environment for cold atom experiments will be outlined. Thanks to the reduced gravity in an earth orbiting satellite, interaction times in the clock can significantly exceed that of fountain clocks on the earth. Several space missions carrying laser cooled cesium and rubidium clocks as well as hydrogen masers are under development for flying onboard the International Space Station in 2006-2007.