This research used data from the 1998 National Educational Longitudinal Study to investigate the development of outcomes for adolescents living with single mothers in multigenerational families compared with adolescents living in married families (as well as a disaggregated set of other family structures). The study measured family structure when students were in 8th grade and examined their developmental outcomes either in 12th grade or 2 years thereafter. Outcomes included self-reported substance use and sexual debut, high school graduation, and college attendance. The disaggregated family structures investigated included never married or divorced single mothers in multigenerational households, two biological cohabiting parents, step families, never married single mothers, divorced single mothers, single mothers with male cohabitors, single father families, grandparent-headed households with no parent present, and married-parent families. Teenagers living in non-married families were less likely to graduate from high school or attend college, more likely to drink or smoke, and more likely to initiate sexual activity. However, teenagers living with their single mother and at least one grandparent in a multigenerational household had developmental outcomes that were at least as good as and often better than outcomes for teenagers in married families. (Contains 37 references.) (SM)