Harold Bluetooth Gormson (Danish Harald Blåtand, Norwegian Harald Blåtann) (ca 911- November 1, 987), sometimes Harold II, succeeded his father Gorm the Old as king of Denmark in 935 (or 940) and king of Norway in 936.
Invading Normandy in 945 in support of Richard the Fearless, Harold's forces took the French king Louis IV prisoner and forced his recognition of Richard's rule. Harold subsequently controlled Norway for a time.
Although his predecessors had accepted Christianity at the instigation of the Frankish Carolingian kings in 826, many Danes and other northerners were still heathens for centuries. Harald Bluetooth was (again ?) forced to accept Christianity, following defeat (972) by the Holy Roman emperor Otto the Great. Otto had already founded many bishoprics including Schleswig, Ribe and Aarhus on the Jutland Peninsula. After his conversion to Christianity, Harold remained a faithful ally of the empire. Otto the Great died in 983 and Harold made his way to the Eider river, but he had to take refuge at Jomsburg in northern Germany when he was fought by not yet christianized Danes. Harold died in battle against the forces of his son and successor Sweyn.
Harold may have had three wives or consorts: Thora, Gunhilde and Gyrid (the niece of the Swedish king Eric the Victorious). He had four children: Håkon, Sweyn, Gunhild and Thyra (who married Styrbjörn Starke).
The Bluetooth interface for wireless Personal Area Networks developed by Ericsson is named after Harold. The Bluetooth logo consists of the Nordic runes for his initials, H and B.