The new Advanced Placement (AP) Computer Science (CS) Principles course increases the need for quality CS teachers and thus the need for professional development (PD). This article presents the results of a 2-year study investigating how teachers teaching the AP CS Principles course for the first time used online PD material. Our results showed that the teaching and computing background of teachers had a significant impact on the teachers' need for and use of online PD material. More specifically, novice CS teachers needed and used PD for developing their pedagogical content knowledge (PCK). Non-CS teachers needed and used PD materials emphasizing content knowledge. Experienced CS teachers believed they had little need for PD even though they were teaching a new course. Our study makes three recommendations for designing effective online PD for CS teachers: match PD to teachers' background, align PD with the course curriculum, and use effective motivational design to enhance teacher engagement.