  In section 2, we introduce fundamental concepts of GR concerning the
measurement of time, relativistic reference systems and we review the recent
literature of chronometric geodesy. In section 3 we introduce the theory of
frequency standard comparisons, beginning with the Einstein equivalence
principle, followed by the description of the frequency techniques, and
finally, we describe clock syntonization and the realization of timescales.
Section 4 describes the geodetic methods for determining the gravity potential,
namely the geometric levelling approach and the GNSS/geoid approach, as well as
considerations about the uncertainties of these methods. In section 5 we
describe the European project ITOC where unified relativistic redshift
corrections were determined for several clocks in European national metrology
institutes. Finally, in section 6 we present numerical simulations exploring
what could be the contribution of clock comparisons for the determination of
the geoid.
