The term light cruiser was given a definition by the Washington Naval Treaty of 1921. The treaty, which sought to limit an arms race in warships, restricted the construction by nations of all large warships. Among its terms, cruisers could displace no more than 10,000 tons standard and light cruisers could be armed with guns of a calibre not exceeding 6.1 inches (155 mm) guns.
By World War I, British light cruisers often had either two 6 inch (152 mm) and perhaps eight 4 inch (100 mm) guns, or a uniform armament of 6 inch (152 mm) guns on a ship of around 5,000 tons, while German cruisers progressed during the war from 4.1 inch (105 mm) to 5.9 inch (150 mm) guns.
In the London Naval Treaty of 1930, light cruisers were defined as cruisers having guns of 6.1 inch (155 mm) or smaller, with heavy cruisers defined as cruisers having guns of up to 8 inch (203 mm). In both cases, the ships could not be greater than 10,000 tons.
In the World War II era, light cruisers had guns ranging from 5 inch (127 mm), as seen in the Atlanta-class anti-aircraft cruiser, to 6.1 inch, though the most common size by far was 6 inch. Heavy cruisers usually had a battery of 8 inch (203 mm) guns. Armament based on guns was considered to be superior to that using guns. The former fired faster and more of them could be carried for the same weight as for the 8 inch. The heavier shell of the 8 inch was little advantage because ships that could withstand a 6 inch hit were well-protected against 8 inch shells, though only the US 6-inch and the Japanese guns had the power to reliably penetrate treaty-cruisers. In the years leading up to World War II, with the London Naval Treaty making it impossible to build a balanced heavy cruiser design within tonnage limits, this led to the construction of a great number of light cruisers of 10,000 ton with twelve to fifteen guns that were otherwise identical to heavy cruisers.
Heavy cruiser construction was phased out in Britain, France and Italy during the mid 1930s. However, the breakout of World War II allowed nations to skirt the London Treaty and exceed the 10,000 ton limit. By the end of the war, the US Navy's ships classed as "large cruisers" with displacements of nearly 30,000 tons (the Alaska class cruiser), while light cruisers stayed in the region of 10,000 tons (although sometimes reaching 12,000 or 13,000 tons). Most modern guided missile cruisers have a similar displacement (10,000 tons for Ticonderoga, 12,000 for Slava, 28,000 for Kirov).
Four light cruisers are still in existence as museum ships, and one is still used in active service by a navy - BAP Almirante Grau of the Peruvian Navy. The four ships preserved as museum ships are: HMS Belfast (1938) in London, HMS Caroline (1914) in Belfast, USS Little Rock in Buffalo, New York, and the more modern Colbert in Bordeaux. Similar ships include the protected cruisers Aurora (St Petersburg) and Olympia, and the bow of the Puglia (Italy).