This volume contains a selection of 99 papers from the International Association of Geodesy (IAG) quadrennial General Assembly, held in Perugia, Italy, from July 2 to 13, 2007, in conjunction with the XXIV General Assembly of the International Union of Geodesy and Geophysics (IUGG). There were 225 oral and 474 poster presentations made at the five IAG Symposia, which covered the following research themes: Reference Frames, Gravity Field, Earth Rotation and Geodynamics, Positioning and Applications, The Global Geodetic Observing System (GGOS). The Symposia focused on Earth observation and the there pillars of geodesy (geometry, Earth rotation, gravity field), and the presented topics spanned a broad and multidisciplinary spectrum of theory and applications. It was clearly evident that, with the recent advances in space missions such as GRACE, geodesy can now contribute to the detection of large-scale temporal phenomena and hence to the solution of important problems of society related to monitoring our environment and understanding global change. Therefore, the title of this volume, 'Observing our Changing Earth', was chosen to reflect this fact, as well as to agree with the general theme of the 2007 IUGG General Assembly, 'Earth: Our Changing Planet'.