We use a construct-based assessment approach to measure learning progression of energy concepts across physical, life, and earth science contexts in middle school grades. We model the knowledge integration construct in six levels in terms of the numbers of ideas and links used in student-generated explanations. For this study, we selected 10 items addressing energy source, transformation, and conservation from published standardized tests and administered them to a status quo sample of 2688 middle school students taught by 29 teachers in 12 schools across 5 states. Results based on a Rasch partial credit model analysis indicate that conservation items are associated with the highest knowledge integration levels, followed by transformation and source items. Comparisons across three middle school grades and across physical, life, and earth science contexts reveal that the mean knowledge integration level of eighth-grade students is significantly higher than that of sixth- or seventh-grade students, and that the mean knowledge integration level of students who took a physical science course is significantly higher than that of students who took a life or earth science course. We discuss implications for research on learning progressions. (Contains 6 tables and 5 figures.)