This book is dedicated to advances in the theory and practice of GNSS reflectometry. It covers a variety of topics in this emerging remote sensing technology, including retrieval of soil moisture, snow depth and snow water equivalent, underground coal mining induced ground subsidence, sea surface height, wind speed and direction, sea surface wave height and rainfall intensity; detection of sea ice, ships, land water body, sea surface rainfall; and data calibration and design of antenna and receiver. It also briefly presents the theory and application of GNSS SAR imaging. The book makes use of ground-based, airborne, and spaceborne data for land and ocean applications especially those related to disasters such as droughts, floods, hurricanes, typhoons, and damaging sea waves. Extensive data processing and experimental results are presented in the major chapters.