This article presents the authors' response to the commentaries on their article. They focus on two important issues raised by most of the reviewers: (a) the curricular dependence of the proposed learning progression and the daunting (twin) challenges of bringing about changes in curriculum and assessment and consolidating and validating the proposed learning progression; and (b) the need for further research and development of a wide variety of sorts. The authors agree that these are major challenges but see them as opportunities as well: opportunities to use research on children's learning to inform curriculum and assessment much more than is currently the case. They argue that although some specific aspects of learning progressions will depend on specific aspects of curricula, which are themselves perhaps a matter of choice and preference, both learning progressions and good curricula are strongly constrained by cognitive and epistemological factors as well. Hence, at a coarse-grained level, there may be fewer truly different paths than might be initially supposed and some general steps in a progression on which all would agree. The authors conclude by underscoring the hypothetical (conjectural) nature of the proposed learning progression, the limitations of the research base that informs it, and the need to do more work that addresses the larger problem of how learning cumulates over a broad sweep of 6 to 8 years.