The underrepresentation of girls from nondominant backgrounds in the sciences and engineering continues despite recent gains in achievement. This longitudinal ethnographic study traces the identity work that girls from nondominant backgrounds do as they engage in science-related activities across school, club, and home during the middle school years. Building a conceptual argument for identity trajectories, the authors discuss the ongoing, cumulative, and contentious nature of identity work and the mechanisms that foster critical shifts in trajectories. The authors argue that the girls view possible future selves in science when their identity work is recognized, supported, and leveraged toward expanded opportunities for engagement in science. This process yields layered meanings of (possible) selves and of science and reconfigures meaningful participation in science. (Contains 2 tables, 1 figure, and 1 note.)