Born in about 1068, the youngest son of William the Conqueror, Henry reigned as king of England between 1100 and 1135, succeeding William Rufus. He received the cognomen "Beauclerc" because he was an educated man, having been given extensive schooling, probably to prepare him for a career as a churchman of some sort.
William's elder sons, William Rufus and Robert 'Curthose', Duke of Normandy, had agreed among themselves that whichever survived the other would inherit both of their father's dominions. In the event, when Rufus died in 1100, in an "accident" while hunting, Robert was on his way back from the First Crusade. His absence, coupled with the fact that (having already tried, and failed, to take England over once (he promoted a rebellion in 1088 but didn't bother turning up with his troops)) he was not popular with the Norman nobility in England, allowed Henry to seize the English treasury at Winchester, gain acceptance by the leading barons, and be crowned at Westminster 3 days after his brother had died, all before Robert knew anything about things.
In November 1100, Henry married Edith (also known as 'Matilda'), the daughter of King Malcolm III of Scotland (the Malcolm of Shakespeare's Macbeth). She was also niece to Edgar Atheling, and therefore a valid link to the pre-Conquest English line of kings (Malcolm had married Edgar's sister). This displeased the Norman baronage, but Edith changed her name to Matilda and this seemed to calm them.
In the following year Robert 'Curthose' tried his luck with an invasion. Defeated, he recognised Henry as king and went home again. Henry, however, did not feel utterly safe and in 1105-6 he returned the favour, defeated Robert, and appropriated Normandy as a possession of England, thus completing the reunion of the Conqueror's territories, as his brothers had all along intended.
During his reign, Henry did much to stabilise England, including confirming liberties and restoring some of the pre-Conquest laws. He established the Exchequer to regulate the collection of revenues, and sought to centralise some of the feudal power by reining in local lords' courts and the excesses of errant sheriffs.
Henry had at least 3 children by Matilda. The two boys, Richard and William, died in the same shipwreck, off the Normandy coast, in 1120; his daughter Matilda married the Holy Roman Emperor, Henry V. He also had many (some say up to 25) illegitimate bairns by other mothers (one, Reinald, became earl of Cornwall; another, Robert of Caen, was Earl of Gloucester and his grand-daughter married earls of Essex and Kent (though not at the same time).
After Matilda's death in 1118, Henry married Adeliza, the 16-year-old daughter of Godfrey, Count of Louvain, but there were no children of this marriage. Left, therefore, without legitimate male heirs, Henry fixed upon the Empress Matilda as his heir, and persuaded the barons to swear to accept her as his heir and successor. He also married her to Geoffrey of Anjou to secure his position in France against the King, Louis VI, and thereby connected the Plantagenets to the English throne.
In 1135, due to a "surfeit of lampreys" (some of which may not have accorded with 20th century standards of edibility, but he had a liking for them), Henry died. His nephew, Stephen of Boulogne, son to the Conqueror's daughter Adela, promptly came to England and laid claim to the throne, on the basis that, being a women, Matilda could not rule. For 18 years war raged between the two, with the Anglo-Norman barons swapping sides as suited them and England was in turmoil.