We introduce Technological Pedagogical Content Knowledge (TPCK) as a way of representing what teachers need to know about technology, and argue for the role of authentic design-based activities in the development of this knowledge. We report data from a faculty development design seminar in which faculty members worked together with masters students to develop online courses. We developed and administered a survey that assessed the evolution of student-and faculty-participants' learning and perceptions about the learning environment, theoretical and practical knowledge of technology, course content (the design of online courses), group dynamics, and the growth of TPCK. Analyses focused on observed changes between the beginning and end of the semester. Results indicate that participants perceived that working in design teams to solve authentic problems of practice to be useful, challenging and fun. More importantly, the participants, both as individuals and as a group, appeared to have developed significantly in their knowledge of technology application, as well as in their TPCK. In brief, learning by design appears to be an effective instructional technique to develop deeper understandings of the complex web of relationships between content, pedagogy and technology and the contexts in which they function.