Learning progressions - descriptions of increasingly sophisticated ways of thinking about or understanding a topic (National Research Council, 2007) - represent a promising framework for developing organized curricula and meaningful assessments in science. In addition, well-grounded learning progressions may allow for coherence between cognitive models of how understanding develops in a given domain, classroom instruction, professional development, and classroom and large-scale assessments. Because of the promise that learning progressions hold for bringing organization and structure to often disconnected views of how to teach and assess science, they are rapidly gaining popularity in the science education community. However, there are significant challenges faced by all engaged in this work. In June 2009, science education researchers and practitioners, as well as scientists, psychometricians, and assessment specialists convened to discuss these challenges as part of the Learning...