This paper presents results of measurements made during the years 1953–56 on the effects of sea-surface reflections on low-angle radars at Xband frequencies. The work was done in three parts and comprised a study of the polarisation properties of sea clutter as a function of sea state, wind direction and incidence angle, a study of the position and depth of interference minima close to the sea surface and a study of the reflecting properties of the sea over a wide range of incidence angles, in so far as these affect the properties of a possible image target. The sea-clutter measurements yielded a well marked polarisation pattern, but the effect of incidence angle did not conform to the conventional linear theory. The interference minima were located close to the positions predicted by the 4/3-earth theory when the aircraft flew at heights of 400ft or more. At lower altitudes, however, the range of the horizon and of the farthest minima considerably exceeded the theoretical values. The peak voltage-reflection coefficients over a range of angles between 1° and 40° relative to glancing incidence very seldom exceeded 0.3 for vertical polarisation and 0.6 for horizontal polarisation. Brewster-angle behaviour was clearly visible for all sea states, and the horizontally polarised reflection was always dominant. In all the results, features were observed for which no theoretical explanations are apparent.