  We review experimental progress on optical atomic clocks and frequency
transfer, and consider the prospects of using these technologies for geodetic
measurements. Today, optical atomic frequency standards have reached relative
frequency inaccuracies below 10-17, opening new fields of fundamental and
applied research. The dependence of atomic frequencies on the gravitational
potential makes atomic clocks ideal candidates for the search for deviations in
the predictions of Einstein's general relativity, tests of modern unifying
theories and the development of new gravity field sensors. In this review, we
introduce the concepts of optical atomic clocks and present the status of
international clock development and comparison. Besides further improvement in
stability and accuracy of today's best clocks, a large effort is put into
increasing the reliability and technological readiness for applications outside
of specialized laboratories with compact, portable devices. With relative
frequency uncertainties of 10-18, comparisons of optical frequency standards
are foreseen to contribute together with satellite and terrestrial data to the
precise determination of fundamental height reference systems in geodesy with a
resolution at the cm-level. The long-term stability of atomic standards will
deliver excellent long-term height references for geodetic measurements and for
the modelling and understanding of our Earth.
