GNSS-R (Global Navigation Satellite Systems Reflectometry) exhibits unique characteristics to complement other active and passive Earth observation methods. One significant attribute is its leveraging of existing and ubiquitous GNSS constellations. This infrastructure assures the reliability and continuity of the transmission signals and supports the foundation for many potential applications in Earth Observation. After first demonstrations of satellite GNSS reflectometry obtained during experiments with UK-DMC satellite, the TDS-1 and CYGNSS missions are now leading this emerging technique to maturity. Geophysical parameters, which can be sensed by GNSS-R, include ocean surface winds, sea surface height, soil moisture, ice properties, snow water equivalent, and vegetation biomass. Building upon the success of GNSS-R, reflectometry-based remote sensing has more recently expanded to use digital communication satellite signals as well.