A key prerequisite for a redefinition of the SI second based on optical atomic clocks is their integration into the international timescales TAI and UTC. This requires a coordinated programme of clock comparisons to be carried out, to validate the uncertainty budgets of the optical clocks, to anchor their frequencies to the present definition of the second, and to establish the leading contenders for a new definition. Such a comparison programme is underway within the EMRP-funded project “International Timescales with Optical Clocks” (ITOC). Illustrated in Fig. 1, this involves four different types of measurement and optical clocks in five different laboratories. Locally, comparisons are being carried out between optical clocks developed in individual laboratories, either by direct beat frequency comparison or by using femtosecond combs to measure optical frequency ratios. To compare optical clocks developed in different laboratories, two different techniques are being explored, both of which have the potential to be applied on an intercontinental scale. Two comparisons will be performed using transportable optical clocks, and an improved two-way satellite time and frequency transfer (TWSTFT) technique based on an increased chip rate is being investigated. In addition to the direct optical clock comparisons, absolute frequency measurements of the optical clocks are also being performed. Several new measurements have already been completed and the current status of the clock comparison programme will be reported at the conference. New methods developed to analyze the self-consistency of the clock comparison data and to derive optimized values for the frequency of each optical clock transition will also be described. To support the clock comparison programme, a complete evaluation is being made of all relativistic effects influencing time and frequency comparisons at the 10-18 level of accuracy, including the gravitational redshifts of the clock transition frequencies. Significant progress has been made towards improved determination of the gravity potential at the sites participating in the optical clock comparisons; gravity surveys have been carried out at all locations and will feed into the computation of a revised European geoid model. Finally, an experiment is in preparation to demonstrate the future impact that optical atomic clocks could have on the field of geodesy. This aims to measure with high temporal resolution the gravity potential difference between two well-defined locations separated by a long baseline (~90 km) and a height difference of 1000 m.