The paper deals with effects of localized obstacles, including reflection as well as shadowing, which are of practical importance in radio communication and radar, particularly on decimetre and centimetre wavelengths. In a digest of available theoretical results, later compared with the experimental findings, attention is drawn to some unresolved difficulties. In most of the practical problems discussed, prediction to a useful approximation is, however, shown to be possible.The experimental work described has been carried out by or for the Admiralty, and much of it had its origin in the special problems of naval communications and radar. It has, however, wider applications at almost every point; applications to radar navigation are specifically discussed.