Atomic time scales TAI and TT(BIPM): present performances and prospects

We review the stability and accuracy achieved by the reference atomic time scales TAI and TT(BIPM). We show that they presently are at the level of a few 10^-16 in relative value, based on the performance of primary standards, of the ensemble time scale and of the time transfer techniques. We consider how the 1 × 10^-16 value could be reached or superseded and which are the present limitations to attain this goal.