Lambert I, Count of Louvain

Count Lambert "the Bearded" (c. 950 - 12 September 1015) was the first person to be described as a count of Leuven (French Louvain) in a surviving contemporary record, being described this way relatively late in life, in 1003. He is also the patrilineal ancestor of all the future counts of Leuven and dukes of Brabant until his descendant John III, Duke of Brabant, who died in 1355.

He fought throughout his life towards the eventual successful establishment of his family in a long-lasting position of power, but Lambert was known throughout much of his life as a rebellious noble, from a rebel family. Lambert was eventually killed in battle at Florennes, fighting against his old enemy Godfrey "the childless", the Duke of Lower Lotharingia who represented royal authority in the region. One monastic writer, Dietmar of Merseburg, described him as the worst person in his whole country ― a country which mourned during his life, and rejoiced at his death.

Historical discussion about the life of Lambert is closely connected to that of his brother and ally Reginar IV. The two brothers entered Lotharingia aggressively from France after the death of their father. They battled for status over decades, and some historians believe that their families only consolidated lasting acceptance after their deaths. Reginar IV, apparently the elder brother, claimed a county based in Mons in Hainaut as something which their father Reginar III had held. The basis of Lambert's claim on Leuven is less clear. It may for example have been granted to him by his father-in-law, the French Carolingian Duke Charles. It is also unclear whether Lambert already laid claim to the Brabant lordship of Brussels within his lifetime. Van Droogenbroeck, for example, has proposed that this territory to the west of Leuven only came to Lambert's family in later generations.