The IAU2000 Resolutions concerning relativity introduce two celestial reference systems a barycentric one BCRS with coordinates (tx) and a geocentric one GCRS with coordinates (TX). The two sets of coordinates are related by 4-dimensional space-time transformations. So far the relations of the BCRS and GCRS with the ICRS and ITRS have NOT been discussed. This will be done here. It is argued that the ICRS is a special represen- tation of the BCRS and the ITRS-coordinates differ from the GCRS ones by a time dependent rotation of spatial coordinates plus possible scale factors. This implies that also the ICRS- and the ITRS- coordinates are related by a generalized Lorentz-transformation.