Students' understanding and proficiency with rational number concepts and operations is considered a key foundational skill for future success in algebra. As middle school students work with these concepts, teachers need timely data to determine whether students are making adequate progress. The purpose of this article is to document the content specifications and technical adequacy data of three experimental measures for Grades 6 to 8 that are intended to provide teachers with ongoing information about students' development of algebra-readiness concepts and skills. Three experimental measures were administered to a total of 575 students in Grades 6 to 8 in two states. Results from a systematic content review indicate that the content assesses key algebra-readiness concepts and skills. Pilot study results suggest that the measures may provide teachers with technically adequate data.