This chapter focuses on challenges in developing and validating a learning progression in the domain of astronomy. These challenges are not unique to astronomy; consideration of them may be useful for researchers working in other areas. As an astronomy education researcher, I believe learning progression research has the potential to provide needed coherence and direction for the field. In general, astronomy education receives a small amount of instructional time (Plummer & Zahm, 2010). Yet current instruction in astronomy, as in other science disciplines, is often fragmented, focused on breadth rather than depth, and with a greater emphasis on inconsequential facts than on the discipline’s core (Kesidou & Roseman, 2002). This suggests that science teachers, in following this fragmented curriculum, may not take full advantage of the limited amount of time allocated to astronomy. In addition, the current research base lacks coherence across conceptual topics and has limited coverage of instructional interventions (for reviews, see Bailey & Slater, 2003; Lelliott & Rollnick, 2010). More work is needed to move astronomy education forward in ways that help teachers, as well as curriculum developers and assessment designers.