Circuitry for synchronizing communications between clock environments wherein a change of state is transmitted from a first clock environment to a second clock environment, the first clock environment being timed by a first clock signal and the second clock environment being timed by a second clock signal, the first and second clock signals having nominally the same frequency but an unknown phase relationship, the circuitry comprising: delay means in the first clock environment arranged to generate a plurality of timing signals by delaying said first clock signal by respectively different delay values; sampling means in the second clock environment for sampling said plurality of timing signals at timing determined by said second clock signal thereby generating a plurality of sampled timing signals; and determining means for generating a control signal based on said plurality of sampled timing signals and outputting said control signal for controlling the transfer time of said change of state.