This study investigated patterns in the development of computational thinking and programming expertise in the context of the Exploring Computer Science (ECS) program, a high school introductory CS course and professional development program designed to foster deep engagement through equitable inquiry around CS concepts. Prior research on programming expertise has identified three general areas of development — program comprehension, program planning, and program generation. The pedagogical prac- tices in ECS are consistent with problem solving approaches that support the development of programming expertise. The study took place in a large urban district during the 2016–17 school year with 28 ECS teachers and 1,931 students. A validated external as- sessment was used to measure the development of programming expertise. The results indicate that there were medium-sized, statis- tically significant increases from pretest to posttest, and there were no statistically significant differences by gender or race/ethnicity. After controlling for prior academic achievement, performance in the ECS course correlated with performance on the posttest. With respect to specific programming concepts, the results also provide evidence on the progression of the development of programming expertise. Students seem to develop comprehension and planning