Harald Sigurdsson (Old Norse: Haraldr Sigurðarson; c. 1015 – 25 September 1066), also known as Harald III of Norway and given the epithet Hardrada in the sagas, was King of Norway from 1046 to 1066. He unsuccessfully claimed the Danish throne until 1064 and the English throne in 1066. Before becoming king, Harald spent 15 years in exile as a mercenary and military commander in Kievan Rus' and chief of the Varangian Guard in the Byzantine Empire. In his chronicle, Adam of Bremen called him the "Thunderbolt of the North".
In 1030, the fifteen-year-old Harald fought in the Battle of Stiklestad alongside his half-brother Olaf Haraldsson. Olaf sought to reclaim the Norwegian throne, which he had lost to Danish king Cnut two years previously. Olaf and Harald were defeated by forces loyal to Cnut, and Harald was forced into exile to Kievan Rus'. Thereafter, he was in the army of Grand Prince Yaroslav the Wise, becoming captain, until he moved on to Constantinople with his companions around 1034. In Constantinople, he rose quickly to become the commander of the Byzantine Varangian Guard, seeing action on the Mediterranean Sea, in Asia Minor, Sicily, possibly in the Holy Land, Bulgaria and in Constantinople itself, where he became involved in the imperial dynastic disputes. Harald amassed wealth whilst in the Byzantine Empire, which he shipped to Yaroslav in Kievan Rus' for safekeeping. In 1042, he left the Byzantine Empire, returning to Kievan Rus' to prepare to reclaim the Norwegian throne. In his absence the Norwegian throne had been restored from the Danes to Olaf's illegitimate son Magnus the Good.