Dō or dou (胴) "breastplate, cuirass" is one of the major components of Japanese armour worn by the samurai and ashigaru or foot soldiers of feudal Japan.

The predecessor of the dō was manufactured in Japan as early as the fourth century. Tankō, worn by foot soldiers, and keikō, worn by horsemen, were both pre-samurai types of early Japanese cuirass constructed from iron plates connected by leather thongs.

During the Heian period (794 to 1185), the cuirass evolved into the more familiar style of armour worn by the samurai known as the dō. Japanese armourers started to use hardened leather along with iron in their construction methods, and lacquer was used to weather-proof the parts. By the end of the Heian period the Japanese cuirass had arrived at the shape recognized as being distinctly samurai. Leather and or iron scales were used to construct samurai armours, with leather and eventually silk lace used to connect the individual scales (kozane) which these cuirasses were now being made from.

In the 16th century Japan began trading with Europe during what would become known as the Nanban trade. Samurai acquired European cuirasses which they modified and combined with domestic armour as it provided better protection from the newly introduced matchlocks known as teppo or Tanegashima from the Portuguese trading Post in the town of the same name. The introduction of the Teppo in 1543 along with a change in battle tactics caused armourers to change the design of the dō from the centuries-old lamellar armour to plate armour constructed from iron and steel plates. This type of armour was called tosei gusoku "new armour". Bullet resistant dō were developed, this type of armour was called tameshi gusoku "bullet tested", allowing Samurai to continue wearing their armour despite the increasing use of firearms.

The warfare of the Sengoku period (15th and 16th centuries) required large quantities of armour to be produced for the ever-growing armies of foot soldiers (ashigaru). Simple munition quality (okashi or lent) dō were massed produced including tatami dō which could be folded.

The victory of Tokugawa Ieyasu at the Battle of Sekigahara in 1600, and his subsequent rise as shōgun in 1603, marked the end of the Sengoku period. By this time Samurai continued to use both plate and lamellar cuirasses as a symbol of their status, but traditional armours were no longer necessary. During the Edo period, lightweight and concealed armour became popular as there was still a need for personal protection. Civil strife, duels, assassinations, and peasant revolts required the use of tatami dō as well as kusari katabira (chain armour jackets) and armoured sleeves as well as other types of armour which could be worn under ordinary clothing. Edo period samurai were in charge of internal security and would wear various types of kusari gusoku (chain armour) and shin and arm protection as well as forehead protectors (hachi-gane). Traditional armour continued to be worn and used in Japan until the end of the Meiji period in the 1860s, with the last widespread use in 1877 during the Satsuma Rebellion.

The type of dō that originally came with a matched suit of armour defined the name for that particular suit of armour, for example, a suit of armour that came with a hotoke would be called a hotoke dō gusoku; a suit of armour that came with a karuta tatami dō would be called a karuta tatami dō gusoku.

True kozane dō are of lamellar construction using individual scales known as kozane, they were used before the introduction of firearms in Japanese warfare.