With the adoption of Common Core State Standards (CCSS) in English Language Arts (ELA) and math and the release and beginning adoptions of the Next Generation Science Standards (NGSS), teachers, schools, and districts are clamoring for professional learning opportunities to refine and re-tool teaching to bring it in line with the reform visions in these documents. This increased need for professional development (PD) raises the question of the capacity of current systems of PD as well as the effectiveness of the most commonly used approaches. The nature of the new demands of these reforms and the scale of the need means that PD will have to use innovative approaches to handle the type of complex learning called for in these reforms, and will have to do so at scale (Wilson, 2013). Many may see online learning environments as part of a solution to address the scale issues, because of the ability to use this technology to reach broad audiences across a wide range of time-frames. There is interest in online or technology-mediated environments that promise certain kinds of functionality to support teachers in the complex work they are being asked to do (U.S. Department of Education, 2010). Wilson (2013) identifies the need to "harness new technologies and social media to make high-quality science PD available to all teachers" as one of the grand challenges in science education. In this commentary, the authors respond to the previous month's article about professional development, praising its contribution but calling for more nuanced consideration of specific program designs to support adoption of a new science curriculum. Specifically, they argue that a corollary to the challenge of access is the challenge of developing research-based design principles to guide the ongoing development, implementation, and evaluation efforts in online PD to meet these new, complex demands in teacher learning.