The American engineer Simon Lake made several contributions to the development of the modern submarine, designing the free-flooding superstructure in 1898. In 1906 German engineers adapted the diesel internal-combustion engine to the submarine. With the development of the periscope and the self-propelled torpedo, the submarine became a formidable factor in naval warfare. The effectiveness of the underwater craft as a deadly weapon was first demonstrated during World War I, when German submarines, known as U-boats, were used extensively against Allied warships and merchant vessels. Their success led to the development of depth charges that were launched from surface vessels.
Between World War I and World War II various improvements were made in submarine design and operation. Underwater sound devices for sonar and ultrasonics were developed for communications and the detection of enemy ships. Rescue devices, such as the lightweight breathing apparatus called the Momsen lung, became standard equipment for crews in case of emergency.

By World War II a typical submarine had a surface speed of about 18 knots using diesel engines and a submerged speed of 8 knots using electric motors. In operating submerged the range was limited by storage-battery power, and the submarine was forced to surface periodically in order to recharge its batteries.

During World War II the German navy developed the snorkel, a device which permits the submarine to recharge its batteries while cruising at periscope depth. The snorkel consists of a long tube extending above the surface of the sea; in the tube are inlet ducts to supply air to the diesel engines and outlet ducts to carry off the engine-exhaust gases. The snorkel increased the underwater range of the submarine enormously. In 1950 a snorkel-equipped submarine set a distance record for underwater navigation by sailing submerged from Hong Kong to Honolulu, a distance of about 8,370 km (5,200 mi) in 21 days.

A new type of hull, shaped like a blimp, was introduced in the USS Albacore, launched in 1953. This hull design proved so successful in providing greater submerged speeds that the teardrop-hull configuration was utilized in nearly all subsequent submarine construction.

In 1954 the British navy launched HMS Explorer, which was powered by turbines using hydrogen peroxide fuel, which greatly extended the underwater range.