"Co-design"--including youth development staff along with curriculum designers--is the key to developing an effective program that is both scalable and sustainable. This article describes Build IT, a two-year afterschool and summer curriculum designed to help middle school girls develop fluency in information technology (IT), interest in mathematics and computer science, and knowledge of IT careers. Build IT is a problem-based curriculum consisting of six units that capitalize on girls' interest in design and communication. The authors outline the need for sustainable, scalable afterschool computer science programs targeting girls, and describe the development of Build IT. Evaluation research on girls' learning of computer science and on the capacity of afterschool staff and organizations to provide computer science programming leads to their description of a research-based approach to sustaining and scaling the program nationally--an approach that other programs might use to expand their reach and impact. (Contains 1 figure.)