Concern about physical inactivity among U.S. youth has been mounting in recent years, in light of studies suggesting that few adolescents (about one out of three) engage in recommended levels of physical activity. Although much attention has been paid to individual factors that may contribute to this problem, such as television viewing and cigarette use, the roles that family and neighborhood influences may play have been largely unexplored. To help fill this research gap, Child Trends analyzed parent report data from the 2003 National Survey of Children's Health (NSCH) to identify family, neighborhood, and individual factors that may have a bearing on adolescent inactivity. One key finding was that, compared with moderately and highly active teens, low-active teens (i.e., those who do not exercise or participate in sports at all) were more likely to have parents who do not exercise. Another finding, not surprisingly, was that low-active teens were more likely than were active teens to be overweight and to spend a lot of time engaged in electronic media such as televisions and computers. Among other findings was that living in a non-supportive neighborhood--one lacking close ties between neighbors as well as overall safety--was associated with whether teens participated in sports but not whether they engaged in exercise. This finding suggests that participation in sports may be more influenced by social or cultural factors than is engaging in exercise. (Contains 2 figures.)