Project Exploration's Sisters4Science (S4S) is an afterschool program for middle and high school urban girls of color. Designed to "get" girls interested in science, "keep" girls interested in science, and "equip" girls with skills and experiences that enable them to pursue science, S4S creates a science-rich learning environment that puts girls at the center. This paper sketches the context for participation in science by girls from historically underrepresented populations and offers a detailed description of S4S and its personalized, girl-centered pedagogy. Project Exploration's personalized curriculum--which focuses on fostering and supporting long-term relationships, knowing students for what they are interested in as well as what they can do, and bringing young people from historically underserved communities together with scientists--is inherent in Sisters4Science. Project Exploration's orientation to making science accessible to urban girls of color is grounded in a girl-centered research base.