Legitimacy, in traditional Western common law, is the status of a child born to parents who are legally married to each other, and of a child conceived before the parents obtain a legal divorce.

Conversely, illegitimacy, also known as bastardy, has been the status of a child born outside marriage, such a child being known as a bastard, a love child, a natural child, or illegitimate. In Scots law, the terms natural son and natural daughter carry the same implications.

The importance of legitimacy has decreased substantially in Western countries since the sexual revolution of the 1960s and 1970s and the declining influence of Christian churches in family and social life.

A 2009 report from the Centers for Disease Control and Prevention indicated that in 2007 a substantial proportion of births in Western countries occurred outside marriage.

England's Statute of Merton (1235) stated, regarding illegitimacy: "He is a bastard that is born before the marriage of his parents." This definition also applied to situations when a child's parents could not marry, as when one or both were already married or when the relationship was incestuous.

The Poor Act 1575 formed the basis of English bastardy law. Its purpose was to punish a bastard child's mother and putative father, and to relieve the parish from the cost of supporting mother and child. "By an act of 1576 (18 Elizabeth C. 3), it was ordered that bastards should be supported by their putative fathers, though bastardy orders in the quarter sessions date from before this date. If the genitor could be found, then he was put under very great pressure to accept responsibility and to maintain the child."

Under English law, a bastard could not inherit real property and could not be legitimized by the subsequent marriage of father to mother. There was one exception: when his father subsequently married his mother, and an older illegitimate son (a "bastard eignè") took possession of his father's lands after his death, he would pass the land on to his own heirs on his death, as if his possession of the land had been retroactively converted into true ownership. A younger non-bastard brother (a "mulier puisnè") would have no claim to the land.

There were many "natural children" of Scotland's monarchy granted positions which founded prominent families. In the 14th century, Robert II of Scotland gifted one of his illegitimate sons estates in Bute, founding the Stewarts of Bute, and similarly a natural son of Robert III of Scotland was ancestral to the Shaw Stewarts of Greenock.