Quantitative reasoning and measurement competencies support the development of mathematical and scientific thinking in children in the early and middle grades and are fundamental to science, technology, engineering, and mathematics (STEM) education. The sixteenth Journal for Research in Mathematics Education (JRME) Monograph is a report on a four-year-long multisite longitudinal study that studied children’s thinking and learning about geometric measurement (i.e., length, area, and volume). This project studied the developing knowledge of continuous quantity in geometric measurement contexts of children from prekindergarten to grade 5. Its second emphasis was an accounting of the research methods and processes used to clarify and revise learning trajectories (LTs). In this study, the research team worked with the same group of children at a Northeast site from pre-K to grade 2, and with a different group of students at a Midwest site from grade 2 through grade 5. Through the results of this study, the authors clarify how children learn and apply measurement knowledge in pre-K through grade 5), in addition to exploring the development of children’s cognitive abilities (e.g., spatial thinking, proportional reasoning) in understanding and using measurement strategies.