Formative assessment is an important strategy for improving student achievement in mathematics. This resource provides tools and examples that allow primary educators to gauge students' knowledge of core mathematics concepts and strengthen their ability to teach effectively. The authors provide 25 formative assessment probes—brief, easily administered activities targeting specific mathematics ideas and concepts—to reveal common understandings and misunderstandings in student thinking. Field-tested with teachers and students at various grade levels, these powerful diagnostic tools help teachers modify their teaching and identify areas that require more instruction.