A 2-wave longitudinal study among 678 early and 317 middle adolescents investigated the applicability of Rusbult’s investment model to adolescent best friendships and tested its usefulness in predicting friendship stability. Results showed that satisfaction, quality of alternatives, and investments predict commitment in friendships, both concurrently and over time. Furthermore, investment model variables predicted friendship stability and, among stable friendships, predicted the tendency to switch best friends. Commitment mediated the effects of satisfaction, investment, and alternatives on tendency to switch. As expected, gender and age differences were found in that alternatives were more important for older adolescents and associations among model variables were stronger for girls. Overall, the investment model proved useful in predicting commitment and stability in adolescents’ best friendships.