Redistribution of mass on the Earth's surface causes changes in the Earth's geometrical shape, gravity field, and orientation in space (the "three pillars of geodesy"), each of which are fundamental considerations for global reference frame definition. On seasonal to several-year time scales, surface mass redistribution is believed to dominate changes in Earth's shape, as measured by stations coordinates of the IGS network. Dynamic models connect this change in Earth's shape to variations in the geocenter (net movement of the solid Earth relative to the center of mass of the whole Earth system), and to Earth rotation. In this lecture I review recent progress in probing these interconnections by analysis of GPS station coordinate time series. The longer term outcome of this new development in geodesy is likely to be a better understanding of connections in the entire Earth system. It will also provide a new approach to comparing different geodetic techniques for consistency through the dynamic models, and it will suggest new ways to combine different geodetic data types for more optimal inversion of Earth models.