The three main radio techniques for remote sensing of the ocean surface, microwave radar, microwave radiometry, and dekametric or HF radar, are surveyed. The generation and characterization of wind waves and swell on the ocean surface, and the first-order mechanism of Bragg resonant scattering of radio waves by such surfaces are outlined. Doppler HF radar techniques, ground wave and sky wave, developed for utilization of this effect to sense remotely wave, wind direction and surface current velocity are reviewed. Second-order scattering effects, hydrodynamic and electromagnetic, are also present, and their signatures on the Doppler echo spectrum of an HF radar permit measurement of wave height, wavelength spectrum and directional spectrum. Finally, the scattering mechanisms relevant in satellite-borne microwave radar are outlined. Bragg resonance occurs with short waves, these being modulated by the longer gravity waves of practical interest. The potential of the radar altimeter, scatterometer and synthetic aperture radar for global ocean surveillance is illustrated by examples from the satellite.