Learning progressions (LPs)--descriptive continuums of how students develop and demonstrate more sophisticated understanding over time--have become an increasingly important tool in today's science classrooms. Here the author discusses some of the research behind learning progressions and presents The Science Inquiry Profile for PreK-4. This is a tool developed by using available research and hundreds of student work samples, which teachers may find useful in assessing students' development of inquiry skills. (Contains 1 figure.)