Concomitant with the technological progress of geodetic observation systems, the accuracy and the temporal resolution of measures of the Earth's varying rotation, i.e., Earth Orientation Parameters (EOP), have steadily improved. In this session we seek contributions in a number of related areas. First, we are interested in the progress of theories of Earth rotation. We seek contributions on theoretical developments that are consistent with the highly accurate observations at the mm-level, to meet the requirements of the IAG’s Global Geodetic Observing System (GGOS). In particular, we invite presentations from the IAU/IAG joint working group, ‘Theory of Earth Rotation’. With respect to geodetic and astrometric observational techniques we seek contributions that highlight new determinations of EOP series and their analyses, including combinations of different observing techniques. We also invite discussions of the dynamical basis for links between Earth rotation, geophysical fluids, and other geodetic quantities, such as the Earth gravity field or surface deformation, and investigations leading to more detailed explanations for the physical excitations of Earth rotation. Besides tidal influences from outside the Earth, the principal causes for variable EOP appear to be related to the changing motions and mass redistribution of the fluid portions of the planet. Observations of the geophysical fluids, such as the atmosphere, oceans, and other hydrological reservoirs, have achieved a new maturity in recent years. Independent observations of the relevant mass fields include the result of recent gravity missions like GRACE. We also welcome contributions about the relationship between EOP variability and current or potential variability in fluids due to climate variation or global change signals. Besides contemporary determination of the EOP and the related geophysical excitations, forecasts of these quantities are important especially for the operational determination of Earth orientation, e.g., for spacecraft navigation; the effort to improve predictions currently is a topic of strong interest. In addition, we will welcome input on the characteristics and variability of the rotation parameters of other planets or planetary bodies.