Across the Western world and in other nations with advanced economies, a remarkable transformation in family systems took place during the final third of the 20th century. The institution of marriage, once nearly hegemonic, lost its nearly universal appeal. Marriage now takes place later in life in virtually all nations with advanced economies, and, not uncommonly, it is delayed indefinitely. New family forms have proliferated gaining legitimacy in the 21st century as alternatives to heterosexual marriage. Specifically, a sharp rise occurred in the prevalence of cohabitation both as a prelude and alternative to matrimony; divorce and remarriage rates have increased in most nations, creating growing family complexity; the legitimation of same-sex unions has changed the form of the family; and, there is a growing level of voluntary childlessness.