The University of Maryland Baltimore County, in collaboration with the University of Maryland College Park, will develop and evaluate professional development activities focused on increasing the expertise of Maryland high school teachers for teaching computer science, with the ultimate goal of increasing the number of rigorous computer science classes offered across the state and the number and diversity of students taking these classes. Experienced higher education faculty and highly effective high school teachers will collaborate to train other high school teachers to develop and offer a college preparatory CS curriculum. In the first year, an initial cohort of 12 master teachers will collaborate with the project leadership team to develop standardized classroom materials based on the CS Principles curriculum and then offer the CS Principles course in their home schools.