The Earth rotation vector links the celestial and terrestrial reference system. The rotation can be separated into (1) nutation and precession and (2) motion of the pole and variations in the length of the day. Based on this separation, the transformations from the terrestrial into the celestial reference frame are described through {\em Earth Orientation Parameters} (EOP). The EOP are determined mainly from VLBI observations. GPS and SLR observations can also be used to determine some of the EOP with high accuracy but with lower long-time stability.
The coordinate transformation between ITRF and ICRF is given by \be x_i^{\rm ICRS} = N_{ik} R_{kl} W_{lj} x_j^{\rm ITRF} \ee where $N$, $R$ and $W$ are time dependent rotation matrices. $N$ gives the movement of the celestial pole (CEP) in ICRS resulting from precision and nutation. The matrix $R$ describes the Earth rotation over the axis from the geocenter to the CEP, and the matrix $W$ describes the polar motion.
Earth rotation observations have a long history starting in the middle of the \cent{19}{th} century with optical methods. After the discovery of latitude variations through K\"uster in 1885, international monitoring of Earth rotation parameters has continued to the present.