Constantine, son of Áed (Medieval Gaelic: Constantín mac Áeda; Modern Gaelic: Còiseam mac Aoidh, known in most modern regnal lists as Constantine II) 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 in northern Great Britain. 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.