In this dissertation, I explored the pedagogical content knowledge of in-service high school educators recently assigned to teach computer science for the first time. Teachers were participating in a professional development program where they co-taught introductory computing classes with tech industry professionals. The study was motivated by three questions: (1) what knowledge of computer science content, student thinking, and instructional strategies do teachers display? (2) what teaching tasks do teachers undertake when planning and implementing their lessons? and (3) how does teaching knowledge relate to the teaching tasks assumed? I conducted a year-long, collective case study with six teachers working in the San Francisco Bay region by gathering interview, questionnaire, observation, and assessment data at monthly classroom visits. Data suggest that (a) teachers know more about student difficulties than instructional strategies specific to computer science; (b) teaching tasks differ in the opportunities they provide for developing teaching knowledge; (c) the way teaching tasks support pedagogical content knowledge varies based on one's level of content knowledge; and (d) teaching confidence and epistemological beliefs influenced instructional decisions but sometimes conflicted with external demands. The findings offer insight into the factors of practice-based training that influence the pedagogical content knowledge development of experienced teachers new to computer science. This work can inform the design of future initiatives to effectively prepare secondary computer science teachers. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]