Michigan State University and Purdue University, in partnership with Project Lead the Way (PLTW), proposes a project entitled "Leading the Way to CS10K: Assessing a Just-in-Time Professional Development Approach for Teacher Knowledge Growth in Computer Science." The project will (i) develop and implement a high-quality professional development approach that incorporates face-to-face training coupled with continuous online just-in-time support at a large-scale; and (ii) assess the effectiveness of that professional development at improving teachers' Knowledge, Skills, and Attitudes (KSAs) for teaching computer science. PLTW is a non-profit organization that has successfully implemented innovative and rigorous STEM curriculum in over 4,700 schools and trained 10,500 STEM teachers across the United States. PLTW is introducing a new Computer Science and Software Engineering (CSE) course, which aligns with the CS Principles framework. The CSE course aims to develop students' computational thinking and introduce computational tools that foster creativity. The large network of schools implementing this new course provides the opportunity to implement and study professional development for a large number of teachers, including teachers with little CS background. This large-scale project will establish an evidence-based professional development program to improve teachers' knowledge to teach computer science, and it will deliver empirically validated best practices for providing computer science professional development to teachers, especially those with limited computer science background.