Boso, brother-in-law to Charles the Bald, succeeded Gerald of Roussillon as count of Lyons and Vienne and declared himself king of Provence in 879. Boso was a generally unsuccessful ruler but he was able to pass the kingdom on to his son Louis in 890. Louis had political ambitions in northern Italy and succeeded in being crowned emperor in 901. However, subsequent fighting in Italy led to his capture by Berengar, a local king, who gouged out his eyes, hence the title 'Louis the Blind.' He ruled Provence for another 20 years, but Hugh of Arles, count of Provence was the dominant figure in the territory. Boso was married to Ermengarde, the only daughter of Emperor Louis II, who named Boso as his heir in Provence.
Though this relationship was on the maternal side, Cardella states that Boso as well as Adrian IV bore the surname of Breakspear.
Adrian sent Boso on a mission to Portugal; for what precise purpose does not transpire, but the fact is attested by the registers of Pope Innocent III.
Boso's name appears attached to many Bulls, both of Adrian IV and of Alexander III.