This article presents a marital horizon theory of emerging adulthood that posits that young people's perceptions of marriage are central factors in determining subgroup differences in the length of emerging adulthood as well as the specific behaviors that occur during this period in the family life cycle. The model was tested with a sample of 813 emerging adults who were recruited from six college sites across the country. Results demonstrated that there are significant differences between young people who have relatively close marital horizons (i.e., those who desire marriage in their early 20s) and those who have more distant marital horizons (i.e., those who desire marriage in their mid-20s or later) in the areas of substance use patterns, sexual permissiveness, and family formation values. Results suggest that changes in lifestyle patterns previously assumed to be associated with the transition to marriage may in fact be initiated when young people anticipate marriage in their near future. (Contains 3 tables and 1 note.)