This article provides teachers with a toolkit for assessing students in the context of data-based individualization (DBI) in mathematics. Assessing students is a critical component of DBI because it provides teachers with information about what they may need to modify in their instructional programs. In this article, we provide teachers with step-by-step procedures for assessing students within the DBI process, including selecting an appropriate progress-monitoring tool, collecting baseline data, and analyzing graphed data to determine whether a student is making adequate progress. In addition, diagnostic assessment is discussed to help teachers target instructional priorities based on student needs. Providing a toolkit for assessment and supplementary resources should enable teachers to use DBI procedures more effectively and, ultimately, to boost students' mathematics achievement.