Long term stability of atomic time scales

We review the stability and accuracy achieved by the reference atomic time scales TAI and TT(BIPM). We show that they presently are in the low 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 We consider how the 1 × 10^-16 value could be reached or superseded and which are the present limitations to attain this goal.