This essay reflects on the challenges of thinking about scale--of making sense of phenomena such as continuous professional development (CPD) at the system level, while holding on to detail at the finer grain size(s) of implementation. The stimuli for my reflections are three diverse studies of attempts at scale--an attempt to use ideas related to professional development in two different countries, the story of how research did or did not frame a nationwide attempt at undergirding CPD, and a fine-grained study of the quality of a dozen mentors' implementation of CPD. The challenge is to "see the forest for the trees," to be able to situate such diverse studies within a larger framework. The bulk of this article is devoted to offering such a framework, the teaching for robust understanding (TRU) framework, which characterizes five fundamentally important dimensions of powerful learning environments. At the most fine-grained level, TRU applies to classrooms, establishing goals for instruction. But, more generally, it applies to all learning environments, and this characterizes important aspects of CPD. The TRU framework thus provides a unifying frame within which one can situate the studies in this volume.