The number of one-parent families has almost doubled since 1975 to nearly two million in 1999. But this does not imply that they constitute a clearly defined statistical grouping, let alone a clearly identifiable social category. The non-existence of a second parent is inferred from the absence of a partner who shares the home, and this involves many approximations. Furthermore, they are far from forming a homogeneous group. While a growing proportion of these families are the consequence of the break-up of a union, there remains a large disparity between parents who were married and later divorced and those who were cohabiting. Parents who never lived in a union and tend to be young are even further removed from widowed parents, who are older and often have grown-up children. These differences play a key role in the greater vulnerability of one-parent families in comparison with couples and their children. Among parents of one-parent families, the youngest mothers, with lower educational levels and higher unemployment, seem diametrically opposed on all points to older mothers and single fathers.