In this "FastFacts," a state requested recommendations about research and practical resources to inform technical assistance conducted with state education staff on formative assessment. The Center on Enhancing Early Learning Outcomes (CEELO) responds by describing what is currently known on this topic. Formative assessment is one key component of a comprehensive assessment system. A Comprehensive Assessment System includes, at a minimum: (1) screening measures; (2) formative assessments; (3) measures of environmental quality; and (4) measures of the quality of adult-child interactions. Formative assessment is defined as a systematic, continuous process used during instruction, that provides a feedback loop to check for progress and detect learning gains, identify strengths and weaknesses, and narrow gaps in teaching and learning. Research has shown that students of teachers who regularly engage in a process of using data to target teaching and student learning demonstrate better learning outcomes compared with students whose teachers do not engage in formative assessment. Formative assessment is critically important to enhance outcomes of students who are struggling with learning, students with disabilities, and students who are English learners. A list of recommended resources is also provided.