Patterns of age at 1st marriage, the stability of marriages, and patterns of marital fertility in 15 developing countries were examined. Survey results for 9 Asian and 6 Latin American countries are included. Formal or informal marital unions and the values surrounding their formation and dissolution are important elements of nearly every social system. Marriage patterns are influenced and influence a wide range of factors, and fertility behavior is 1 of the most important factors. Due to this link people interested in achieving lower fertility levels in developing countries may view manipulation of marriage patterns as a potentially useful means of realizing that goal, yet the relationship between marriage and fertility is complicated. Evidence exists that in most of the Asian countries there is some trend toward later marriage, but there is much less evidence of any such trend in the Latin American countries. Informal consensual unions are widespread in Latin America and exhibit 2 basic patterns: numerous consensual unions among younger women gradually outnumbered by formal marriage among older women versus a relatively steady but fairly low proportion of consensual unions at all ages. Survey results support the idea that women with more education or with urban backgrounds tend to marry later. Major differences exist in the extent of divorce and separation. A country's birthrate may be reduced by lowering the level of either marital fertility rates or the proportions married. Family planning programs are aimed at the fertility rates, but changing the proportion of women married cannot be ignored. Social change is always difficult to bring about, and efforts to change marriage patterns are no different.