This volume opens up new perspectives on the physics of the Earth's interior for graduate students and researchers working in the fields of geophysics and geodesy. It looks at our planet in an integrated fashion, linking the physics of its interior to the geophysical and geodetic techniques that record, over a broad spectrum of spatial wavelengths, the ongoing modifications in the shape and gravity field of the planet. Basic issues related to the rheological properties of the Earth's mantle and to its slow deformation will be understood, in both mathematical and physical terms, within the framework of an analytical normal mode relaxation theory. Fundamentals of this theory are developed in the first, tutorial part. The second part deals with a wide range of applications, ranging from changes in the Earth's rotation to post-seismic deformation and sea-level variations induced by post-glacial rebound. In the study of the physics of the Earth's interior, the book bridges the gap between seismology and geodynamics.