The Children's Measurement Project examines children's developing knowledge from PreKindergarten through Grade 5 as they develop the capacity and strategies they need to measure geometric space (length, area and volume), investigating number concepts, early algebra, or variability. We investigate ways children learn to use measures as evidence for scientific or mathematical claims. We began by examining the literature on learning trajectories and progressions to interpret existing research on children's understanding of length, area and volume. Our work engages both Rasch modeling and learning/teaching experiments within clinical and classroom contexts to collect data for longitudinal accounts of children's development of measurement concepts and strategies. The work is being conducted as a collaboration of Illinois State University and the University at Buffalo (State University of New York). We are beginning the fourth year of our project (2010).