In the first section of the chapter we give an account of the basic traits of the Earth rotation theory, starting from the assumption of a rigid Earth and solving Euler’s equations. The Liouville equations that express momentum conservation, valid for a deformable Earth, are linearised for small excursions of the pole of rotation and solved for polar motion and for the length of day by means of a perturbative approach. Then, the rotational and the surface loading excitation functions are obtained in a closed form, in terms of centrifugal potential variations and of surface loading functions, respectively. In the second section, we deal with the form of the linearised Liouville equations that is suitable on the time-scales of the GIA process. Two distinct Earth rotation theories—referred to as traditional and new, respectively—are presented, based upon the existing literature. This allows us to obtain a general form of the polar motion transfer function, leading to a general solution of the Liouville equations in a form suitable for GIA investigations. The third section is devoted to the study of the Rotation Response Functions (RRFs), which represent the rotational counterpart of the SRFs. The RRFs stem from a time-convolution between a rotation GF and the variation in the centrifugal potential due to polar motion. Expressions for the RRFs associated to the displacements and to the geoid height variation are determined in a closed form, taking advantage of the solution of the Liouville equations for GIA.