Work package leader: Dr Marco Pizzocaro, INRIM Other participants: CMI, LNE, NPL, OBSPARIS, PTB, UMK, VTT The aim of this work package is to reduce systematic uncertainties through improved understanding and control of dominant frequency shifts. Clock accuracies of 1 × 10−18 are targeted through control of blackbody radiation (BBR) shifts from the atoms’ thermal environment, light shifts from the optical lattice, and collisions within lattices. The BBR shift uncertainty in most optical clocks is dominated by uncertainty in the thermal field at the position of the atoms. This uncertainty can be reduced through better characterisation of the thermal environment. Additionally, this project will pursue an alternative approach to reducing the shift and the uncertainty by interrogating the atoms inside a cryogenically cooled enclosure. Yb lattice clock from INRIM Other important terms in the uncertainty budget of optical lattice clocks are (higher order) light and collision induced shifts. Theoretical proposals have been made for possible strategies for Sr, Yb, and Hg clocks to achieve lattice-induced systematic uncertainty below 1 × 10−18 and these will now be investigated experimentally. Collisional shift cancellation has already been demonstrated elsewhere below 1×10-18 for Sr and at the 5×10-18 level for Yb and this project will now target achieving 1×10-18 uncertainties in European clocks for both Sr and Yb systems.