U-boats had several methods for avoiding detection, both from radar and from sonar/asdic (the term used by the RN at the time). These included tactics, the design of the craft and special decoys and detectors.

When U-boats were submerged, the main method used for detecting them was active sonar, called asdic by the RN. This produced pulses of high-frequency sound waves; these bounced off the U-boat, and could be detected back at the ship. By timing the return pulses, the distance to the U-boat could be measured. U-boat captains soon learned the limitations of this. Asdic gave the greatest effect when the target was broadside on to the detector. As such, its effect could be reduced by pointing the bow or stern of the U-boat towards the detector. Diving deep reduced the effectiveness of asdic, so U-boat captains tended to dive as deep as they could when attacked. The asdic mountings also tended to lose contact with the target at close range. U-boat captains used this to their advantage, making radical manoeuvres when ships closed in to attack. From 1942, the U-boat force was equipped with a sonar decoy system. Called Bold (kobold in English), it ejected small canisters of chemicals. These reacted with seawater to form a cloud of bubbles, which would reflect sound much like a U-boat. Typically, the decoys would be deployed while the submarine was making evasive maneouvres. The submarine would make several hard turns, deploying a decoy after each turn. Finally, the asdic signature of a submarine could be reduced by giving it a coating that absorbed, rather than reflecting, sound. These 'anechoic' coatings were tested aboard U-11 in 1940, using a rubber-byproduct called 'Oppanin'. They first entered active service with U-67 on a trial basis, but there were problems keeping the rubber tiles attached to the hull. This was not solved until the Type XXI and XXIII U-boats were entering service in 1945. Few of these boats saw actual service.

The sounds produced by a U-boat could also be detected, using passive sonar or hydrophones. The sound produced by a U-boat was known as its 'hydrophone effect'. The main contributor to this was the engine and propellor noise of the U-boat. This could be reduced simply by moving at slow speeds, speeding up only to avoid attacks. Sticking behind escorts, in the sound shadow created by their propellers and engines, could prevent detection. The amount of sound a U-boat produced could be reduced by careful design work. Machinery, switches and other noise-producing equipment were mounted on rubber mounts. This was mainly intended to absorb the shock of nearby depth-charges, but also provided some sound insulation. The rubber prevented the vibrations of the machinery being transmitted into the hull of the U-boat, and from there into the water. Keeping the crew quiet, and stopping them from working also helped. U-boat sound insulation was generally not quite as intensive as on British boats, but was still somewhat effective. Finally, a decoy system was produced. Called 'Siegelinde', it was essentially a small torpedo. Launched out of the same tubes as the Bold decoy, it would run in circles, making a similar noise to a U-boat's engines. It also contained the same chemicals as Bold, creating an asdic decoy that moved, rather than staying still. Another decoy system, called Siegmund, used explosive charges to deafen hydrophones and asdic receivers.

U-boats on the surface could be detected by radar, either from surface ships or from aircraft. Avoiding this was as simple as diving, but this required warning of a radar detection. This needed special radar detectors. The first of these was introduced in 1942, and called Metox (or Wanz, when used with an aerial capable of automatic scanning). This was tuned to pick up the 1.5m wavelength used by the ASV II radar used by British aircraft in the period, but could also detect the emissions of surface ships using radars with similar wavelengths. Metox was fairly effective as long as the Allies used radars with metric wavelengths. However, in 1943, they switched to centimetric wavelengths, rendering Metox almost useless. The number of air attacks on surfaced submarines rose massively. A captured airman informed the Germans that this was the result of the Allies detecting and homing in on emissions from Metox. They checked that this was technically possible, found that it was, and instructed U-boat commanders to limit their use of Metox. Rather than stopping the attacks, it just reduced the U-boat's defence against them. Eventually, though, it became clear that the Allies were using centimetric radars, leading to the development of the Naxos receiver. This was ineffective at first, as the Germans used the wrong polarisation in the receiver. After this was fixed, it was fairly effective, until the Allies introduced a new model of ASV radar. This allowed them to change the strength of the radar signal, making it look like an aircraft hadn't detected a submarine when it was actually closing in to attack. Towards the end of the war, they introduced radars using the 3cm frequency band, which Naxos could not detect. A radar detector at this frequency was developed for the Luftwaffe, but hadn't made it to the U-boat service before the end of the war. There were also two types of radar decoys. Aphrodite was a hydrogen balloon, with a sea anchor to keep it close to the surface of the sea and foil strips attached to give it a radar signature. Filled from hand bottles, it could be thrown over the side. It had a lifespan of about six hours. Thetis, meanwhile, was a buoy which floated on the surface of the sea. U-boats carried between 15-30, and would throw them overboard as they crossed the Bay of Biscay, filling the area with decoy targets. They were less effective than Aphrodite, but had a longer life.

There were also a number of decoys used to convince attacking escorts that they had sunk the submarine. A special oil canister was developed. This could be ejected much like a Bold canister. It formed an oil slick on the surface, one of the indications that escorts used to confirm a U-boat sinking. Some captains also chose to eject rubbish, wreckage, cap ribbons and the like. These added to the oil slick, suggesting that the U-boat had sunk. The hope was that the attacking ships would see these, assume the submarine had been destroyed, and give up.