In this paper, we report on our efforts to support teachers' development of mathematical knowledge for teaching through online professional development focusing on authentic engagement in collaborative mathematical problem solving. In particular, this paper seeks to explore and understand the relationship between online mathematical collaboration and the development of Mathematical Knowledge for Teaching. Results indicate that particular online mathematical collaboration, including individual mathematical problem solving, mathematical collaboration, discussion in groups, and emergent conversations focusing on issues of teaching and learning in the context of the teachers individual and collaborative work, holds the potential for supporting the development of Mathematical Knowledge for Teaching. The specifics of these results will be presented and potential implications for online mathematics teacher professional development will be discussed. (Contains 2 figures and 1 note.)