A formative pilot study of a fractions intervention was conducted to address the intervention's potential for improving fifth-grade struggling students' knowledge of fractions, and to identify any logistical or instructional design issues, before scaling up for a randomized control trial. In the first section, we provide details of the formative pilot study and present preliminary data that demonstrate the potential of the intervention. Intervention lessons--adapted from a commercially available program--emphasize consistent use of the number line in conjunction with concrete representations to scaffold learning and facilitate understanding of grade-level and foundational fractions content. We explain how intervention revisions were guided by the formative evaluation. In the second section, we present key concepts that distinguish understanding fractions principles from understanding whole number principles. These concepts relate to understanding foundations of fractions (e.g., magnitude, equivalence, part-whole, and measurement) and the four operations. We present these key concepts and show how the fractions intervention addressed them as a guide to practitioners for teaching new and difficult fractions content, as students integrate fractions understanding with their understanding of whole numbers.