(auteur) On the occasion of the thirtieth anniversary of the International Earth Rotation and Reference Systems Service (IERS), this paper reviews the progress that has been made in the development of the International Terrestrial Reference Frame (ITRF). A particular focus will be devoted to (1) the frame definition (origin, scale, orientation and their time evolution) and how it is maintained over time, (2) technique systematic errors and their impact on the frame definition, (3) the possible ways of dealing with station non-linear motions, and (4) the level of agreement between techniques at co-location sites. Improvements planned for the future ITRF releases will be outlined taking into account user needs in both operational geodesy and Earth science applications.