The co-evolution of adolescents' friendship networks and their smoking behavior is examined in a large sample across six European countries. Selection and influence processes are disentangled using new methods of social network analysis that enable alternative selection mechanisms to be controlled for. The sample consisted of 7704 adolescents participating in the control group of the ESFA (European Smoking prevention Framework Approach) study. The design was longitudinal with four observations. The main measurements were friendship ties, adolescents smoking behavior, parental smoking behavior, and sibling smoking behavior. Results indicated that in each country adolescents preferred selecting friends based on similar smoking behavior. Support for the influence of friends was found in only two countries. A similarity in smoking behavior between friends was explained more strongly by smoking-based selection processes than by the influence of friends in each of the six countries. Prevention programs need to address aspects that drive peer selection, and reinforce non-smoking attitudes in adolescents.