This case study focuses teacher education toward the design of online learning educational environments to guide in-service teachers’ development of technological pedagogical content knowledge (TPACK), the knowledge for effectively integrating technologies in their classroom instruction. This study describes a researcher-designed learning trajectory instructional approach that highlights key online instructional features that guide teachers in improving and refining their TPACK. In an online Masters’ degree program, the research-based learning trajectory transforms teachers’ knowledge for teaching mathematics with technologies by focusing on the development of their knowledge-of-practice through “systematic inquiry about teaching” with technology that considers “learners and learning, subject matter and curriculum, and schools and schooling” [6]. The multiple case, descriptive study provides a rich description of how the features of the learning trajectory influence nine K-12 teacher participants’ thinking about their own thinking with the technology for learning mathematics and their thinking about their students’ thinking and understanding when learning with multiple technologies. The study concludes by proposing that teacher educators consider incorporating such a researcher-designed learning trajectory instructional approach to interweave descriptive tasks with specific pedagogical strategies in order to enhance teachers’ knowledge for teaching their content with technology – their TPACK.