This article explores one segment of an extended research and development project that was conducted to better understand the ways online teacher professional development can support teachers' development of deep and connected mathematical understandings. In particular, this article discusses teachers' understandings of the concept of function--primarily functions as a process and covariational reasoning--and how specific online professional development activities can support gains in teachers' understandings of the foundational reasoning abilities and understandings that underlie pre-calculus and calculus. The authors present the mathematical goals for the professional development, the mathematical tasks participating teachers engaged with, and the model for online collaboration employed to support teachers' mathematical development. The article includes a discussion of the significant gains that can be attributed to participation in the online professional development and the implications for mathematics education.