Use of frequency standards in TAI: Present and future

In this paper we study the performance of the ensemble of high accuracy primary frequency standards (PFS) that ensure the accuracy of International Atomic Time TAI. We analyze the statistics of the results of PFS evaluations compared to their stated uncertainties and we discuss the different components of the uncertainty. We also consider the use of secondary frequency standards for the same purpose. Finally we envision the evolutions brought by a coming generation of new ultra-accurate standards and the problems that need to be solved for their future usage in TAI.