This five-wave study aims to investigate the development of adolescents' perceptions of support, negative interaction, and power in best friendships from ages 12 to 20 years. Furthermore, gender differences and linkages between the three dimensions are explored. A total of 593 early adolescents (53.6% boys) and 337 middle adolescents (43.3% boys) participated. A multigroup multivariate accelerated growth curve showed an increase of support for both boys and girls. Negative interaction was found to temporarily increase and then decrease for boys, while remaining stable for girls. Power temporarily increased for boys and decreased for girls. Results indicated that (1) friendships become more supportive during adolescence, (2) power issues are more prominent in friendships of boys and more powerful peers are perceived as more supportive by boys but not by girls, and (3) friendships of boys show a lagged development towards more equality.