This is the first authored English book completely focused on global navigation satellite system reflectometry (GNSS-R). It consists of two main parts: the fundamental theory; and major applications, which include ocean altimetry, sea surface wind speed retrieval, snow depth measurement, soil moisture measurement, tsunami detection and sea ice detection. Striking a healthy balance between theory and practice, and featuring in-depth studies and extensive experimental results, the book introduces beginners to the fundamentals, while preparing experienced researchers to pursue advanced investigations and applications in GNSS-R.