Abstract: The International Terrestrial Reference Frame (ITRF) has been regarded as the most important global terrestrial
reference frame that has the most rigorous theoretical framework, adopts the most advanced approaches and achieves the highest
accuracy. Other global and regional TRFs usually use ITRF to determine their datum. Based on the history and development of ITRF
introduced in this paper, we overview the status and problems of ITRF from these three aspects: mathematical models of
multi-technique combination, definitions and realizations of TRF datum and space-geodetic time series. As a conclusion, we give
some recommendations to establish TRF at mm-level in the near future.

Key words: terrestrial reference frame at mm-level, geocenter motion, multi-technique combination, seasonal variation