Radar returns from the ocean surface have been observed since the earliest days of radar. They were characterized as “clutter” because they often obscured targets, such as ships or aircraft. However, Crombie (1955) observed that some high-frequency (HF, 3-30 MHz) signals recorded near the sea had a distinctive Doppler shift of a fraction of a hertz above and below the transmitted signal. He correctly deduced that they were the result of Bragg scattering by ocean waves that were traveling radially toward or away from the radar and had a wavelength of one-half the radar wavelength. That observation launched the field that is now termed “radar oceanography,” the use of radar systems to study oceanographic properties. Radar systems can be characterized by a number of parameters including operating frequency, geometry, platform, propagation mode, means of obtaining distance and angular resolution, etc. In the limited space of this paper only a few of the highlights of HF systems can be covered. For in depth reviews, the reader is referred to articles by Croft (1972), Barrick (1978), and Shearman (1981, 1983).