This primer argues that effective formative assessment is essential to successfully implementing new college- and career-ready standards. It explains what formative assessment is, how it works in practice, and why it is critically important in fostering powerful pedagogy and 21st-century competencies. It then contrasts the purposes and uses of formative assessment with those of other forms of assessment in a comprehensive and balanced assessment system. It also offers recommendations for policymakers at state and local levels in how to support formative assessment, and not unintentionally undermine it. Ultimately, this primer argues for formative assessment’s central role in fostering a culture of learning for students and teachers.