Harald was the son of Gorm the Old and his wife Thyra. Harald became King of Denmark when his father died in 958 or so. While Gorm is usually credited with being the first king of Denmark, it is unclear how much territory he actually controlled and his kingdom may only have consisted of the Jutland peninsula. Harald consolidated his control over Jutland and Zealand, plus portions of what is now Norway and Sweden.
Harald introduced Christianity to Denmark after his own baptism around 960. He erected the rune stones at Jelling which included this inscription about Harald:
“King Harald bade these memorials to be made after Gorm, his father, and Thyra, his mother. The Harald who won the whole of Denmark and Norway and turned the Danes to Christianity.”
Harald is also known for building ring forts in at least five locations within Denmark, including Aarhus. With relative quiet within his realm, Harald was always interested in foreign adventures. Following the assassination of his nephew King Harald Greycloak of Norway in 970, Harald became king of Norway, though he left most of the management of Norway to his vassal Haakon Sigurdsson.
Harald was married three times. His son and heir Sweyn Forkbeard was born around 960 and is believed to be the son of Harald’s first wife Gunhild. Sweyn rebelled against his father during the last years before Harald’s death. It is not clear how Harald died, but some sources claim that Sweyn deposed his father before his death.
As an aside, the Bluetooth wireless technology was named after Harald Bluetooth in 1997. Apparently the inventors felt that Bluetooth would unite wireless devices in the same manner that Harald had united Denmark.