The challengeI begin with a metaphor to characterize the challenges that authors and readers face when confronting issues of scale. Consider the ways that map makers use insets-devices that allow for a focus on particular areas, while situating those areas within the larger whole. The use of insets allows for characterizations at two orders of grain size. Now imagine a map of the world containing four insets. Two of those insets provide details about train service in a small number of towns in England and the United States. A third provides information about why bus lines are organized the way they are in Sweden, and the fourth describes the ways in which a dozen automobile drivers in Germany made use of new technology-enhanced automobiles. Your challenge, as a map-reader: use this information to think about the design of coherent transportation systems, worldwide.That's quite a challenge. The point is that such insets, which serve to provide local descriptions of different aspects of a large and complex phenomenon, will only get one so far in understanding the issues at hand. The same is the case when one reads the chapters by Clark-Wilson, Hoyles, Noss, Vahey, and Roschelle (2015), Boesen, Helenius and Johansson (2015), and Kuzle and Biehler (2015), with an eye toward understanding the landscape of continuous professional development (CPD).At the level of the individual papers, one can note interesting findings and point to aspects of the papers worth following up on, along various dimensions (e.g., with regard to theory, method, and practice). For example, Clark-Wilson, Hoyles, Noss, Vahey, and Roschelle's (2015) choice of methods-to use questionnaires, fleshing out the picture with two case studies-highlights a way to operate at two levels of scale. At the same time, there is much that the paper does Abstract 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.