Data from almost 13,000 respondents to the National Longitudinal Study of Adolescent Health ("Add Health") are used to examine the impact of residential and school mobility on the structure of adolescents' friendship networks and the degree to which parents know their children's friends and the parents of those friends. Recent movers or school changers tend to have small, dense networks, and to occupy less central and less prestigious positions in their networks, and the parents of mobile adolescents are less knowledgeable about members of their children's networks. These effects appear to persist for several years. The level of mobility in the school often has an independent impact on the character of adolescents' friendship networks; students in high-mobility schools have smaller networks and receive comparatively few friendship nominations, and their parents are less likely to know their children's friends and those friends' parents. The negative impact of individual mobility on some dimensions of adolescents' friendship networks is attenuated by high levels of mobility in the adolescents' schools. The impact of mobility on some network characteristics is especially pronounced among older adolescents and among girls.