International Atomic Time (TAI) is a realization of Terrestrial Time (TT), stable at the level of 4 x 10<SUP>-16</SUP> over one month. It is computed at the International Bureau of Weights and Measures (BIPM) using clock and primary/secondary frequency standards' data that are contributed by metrology institutes, observatories and space agencies. TT(BIPM) is computed for applications requiring long-term stability, such as pulsar timing, using the average of the evaluations of the frequency of TAI by the primary frequency standards. The international reference time scale, Coordinated Universal Time (UTC) is derived from TAI. UTC is realized at the institutes contributing data to the BIPM, and disseminated by them for scientific and civil applications.Astronomers need to access time scales of different kinds, and consequently benefit from the progress of time and frequency metrology. A dozen astro-geodetical observatories and stations realize time scales traceable to UTC for internal applications, and programs are conducted for the observation of millisecond pulsars aiming at the maintenance of an astronomical, dynamical time scale.Several factors have contributed in recent years to the improvement of time scales: the operation of a new generation of primary standards, the cesium fountains, and the implementation of refined algorithms allowing a better statistical treatment of the data, with impact on the stability. The development of optical combs and ultra-stable lasers is exploited in optical frequency standards, which are starting to reach systematic errors two orders of magnitude smaller than today's primary standards. Reaching 10<SUP>-18</SUP> accuracy raises new challenges, notably in performing long-distance clock comparisons and in correcting for relativistic effects. Fibre-optic links are being developed around the world and display excellent performances for time and frequency comparisons over continental distances. Coordinated efforts are also expected in the application of astronomical observation techniques such as VLBI and laser ranging to these very accurate time and frequency comparisons.