The primary purpose of this study was to investigate the reliability, validity of Algebra Foundations and Content Analysis-Multiple probes and to examine the utility of these probes in monitoring student progress over a full school year. In addition, we also examined the use of a third type of probe, Basic Skills, with a small number of students. Our findings revealed that both the Algebra Foundations and Content Analysis-Multiple Choice probes possessed adequate levels of alternate form and test-retest reliability. We examined two types of validity: concurrent and predictive validity. Concurrent validity was assessed by investigating the relationship between probe scores and other indicators of proficiency in algebra including teacher proficiency ratings and standardized test scores. In general, we found probe scores were associated with standardized test scores for students in grades 9-12, but not with teacher ratings of proficiency or standardized test scores for eighth grade students. The predictive validity of the probes was assessed by examining the association between probe scores and other indicators of proficiency including teacher ratings of growth and standardized test scores. Our findings were identical to those for concurrent validity. When examining student progress over time, we found that the Content Analysis-Multiple Choice probes were more sensitive to reflecting student growth than were the Algebra Foundations probes. When investigating student progress over time by class type, we found that only 8th Grade Algebra students showed .5 unit weekly growth on both probes; Algebra 1 students had a mean slope value near this threshold (.47) on the Content Analysis-Multiple Choice probes. This result indicated that the utility of the probes for monitoring student growth may differ for students of different mathematics ability levels.