Constantine, son of Áed (Medieval Gaelic: Causantín mac Áeda; Modern Gaelic: Còiseam mac Aoidh, known in most modern regnal lists as Constantine II; born no later than 879; died 952) was an early King of Scotland, known then by the Gaelic name Alba. The Kingdom of Alba, a name which first appears in Constantine's lifetime, was situated in modern-day Scotland.
The core of the kingdom was formed by the lands around the River Tay. Its southern limit was the River Forth, northwards it extended towards the Moray Firth and perhaps to Caithness, while its western limits are uncertain. Constantine's grandfather Kenneth I of Scotland (Cináed mac Ailpín, died 858) was the first of the family recorded as a king, but as king of the Picts. This change of title, from king of the Picts to king of Alba, is part of a broader transformation of Pictland and the origins of the Kingdom of Alba are traced to Constantine's lifetime.
His reign, like those of his predecessors, was dominated by the actions of Viking rulers in the British Isles, particularly the Uí Ímair ("the grandsons of Ímar", or Ivar the Boneless). During Constantine's reign the rulers of the southern kingdoms of Wessex and Mercia, later the Kingdom of England, extended their authority northwards into the disputed kingdoms of Northumbria. At first allied with the southern rulers against the Vikings, Constantine in time came into conflict with them. King Æthelstan was successful in securing Constantine's submission in 927 and 934, but the two again fought when Constantine, allied with the Strathclyde Britons and the Viking king of Dublin, invaded Æthelstan's kingdom in 937, only to be defeated at the great battle of Brunanburh. In 943 Constantine abdicated the throne and retired to the Céli Dé (Culdee) monastery of St Andrews where he died in 952. He was succeeded by his predecessor's son Malcolm I (Máel Coluim mac Domnaill).
Constantine's reign of 43 years, exceeded in Scotland only by that of King William the Lion before the Union of the Crowns in 1603, is believed to have played a defining part in the gaelicisation of Pictland, in which his patronage of the Irish Céli Dé monastic reformers was a significant factor. During his reign the words "Scots" and "Scotland" (Old English: Scottas, Scotland) are first used to mean part of what is now Scotland. The earliest evidence for the ecclesiastical and administrative institutions which would last until the Davidian Revolution also appears at this time.
Compared to neighbouring Ireland and Anglo-Saxon England, few records of 9th- and 10th-century events in Scotland survive. The main local source from the period is the Chronicle of the Kings of Alba, a list of kings from Kenneth MacAlpin (died 858) to Kenneth II (Cináed mac Maíl Coluim, died 995). The list survives in the Poppleton Manuscript, a 13th-century compilation. Originally simply a list of kings with reign lengths, the other details contained in the Poppleton Manuscript version were added in the 10th and 12th centuries. In addition to this, later king lists survive. The earliest genealogical records of the descendants of Kenneth MacAlpin may date from the end of the 10th century, but their value lies more in their context, and the information they provide about the interests of those for whom they were compiled, than in the unreliable claims they contain.
For narrative history the principal sources are the Anglo-Saxon Chronicle and the Irish annals. The evidence from charters created in the Kingdom of England provides occasional insight into events in northern Britain. While Scandinavian sagas describe events in 10th-century Britain, their value as sources of historical narrative, rather than documents of social history, is disputed. Mainland European sources rarely concern themselves with affairs in Britain, and even less commonly with events in northern Britain, but the life of Saint Cathróe of Metz, a work of hagiography written in Germany at the end of the 10th century, provides plausible details of the saint's early life in north Britain.
While the sources for north-eastern Britain, the lands of the kingdom of Northumbria and the former Pictland, are limited and late, those for the areas on the Irish Sea and Atlantic coasts—the modern regions of north-west England and all of northern and western Scotland—are non-existent, and archaeology and toponymy are of primary importance.