Surprise has always been an important element of military tactics, and electronics must make its contribution to the element of surprise. It does this by radar, and by the various countermeasures that can be used against radar. The paper discusses the parallel development of radar and electronic countermeasures and shows how the battle between them has worked with the laws of physics to define modern radar and countermeasure systems. Radar was first used to see approaching enemy aircraft at long range, so that fighters could ambush them. Radar allowed its user to see the enemy and to predict his movements, without being detected himself. The initial advantage of the radar was reduced by the discovery that the radar's own transmissions could be heard at much greater ranges than those at which it could detect its targets, and that it was capable of being deceived by false targets. These two principles lie at the heart of the art of countermeasures. The subsequent development of military radar has been much concerned with countering these countermeasures, which have themselves grown progressively more sophisticated. The importance of the parasitic relationship between the two is emphasised. The paper describes two of the latest developments: channelised receivers that can intercept many radars simultaneously, and the PILOT radar which has been specifically developed to make its transmissions virtually indetectable.