This starts with a historical overview of Earth Observations using GNSS signals. Next the basic physics of using GNSS-R is reviewed, followed by a description of the observables. Next, the techniques are reviewed, followed by a discussion of the instrumentation that can implement these techniques. Then, applications are given in three areas: Ocean, Land, and Cryosphere. Next, an overview of an experiumental 6U cubesat mission is given, followed by a view of the future.