The idea for organl.zl.ng an Advanced Research Workshop entirely devoted to the Earth rotation was born in 1983 when Professor Raymond Hide suggested this topic to the special NATO panel of global transport mechanism in the Geosciences. Such a specialized meeting did not take place since the GEOP research conference on the rotation of the Earth and polar motion which was held at the Ohio State University (USA) in 1973. In the last ten years, highly precise measurements of the Earth's rotation parameters and new global geophysical data have become available allowing major advance to be made in the under standing of the various irregularities affecting the Earth's rotation. The aim of the workshop was to bring together scientists who have made important contributions in this field during the last decade both at the observational and geophysical interpretation levels. The confe rence was divided into four main topics. The first session was dedicated to the definition, implementation and maintenance of the terrestrial and celestial reference systems. A few critical points have been identified as requiring further improvements: (i) appro priate selection of terrestrial sites recognized for their long term stability, (ii) determination of the relationship between terrestrial and celestial references systems as well as between the various terrestrial ones, (iii) improvment of the theory of a rotating elastic earth (the recently adopted theory needs already some corrections').