To many of today's teachers, assessment is synonymous with high-stakes standardized tests. It is often viewed as something in competition with teaching, rather than as an integral part of teaching and learning. In the current accountability environment, assessment is not regarded as a source of information that can be used during instruction. Instead, it has become a tool solely for summarizing what students have learned and for ranking students and schools. But, there is an entirely different kind of assessment that can actually transform both teaching and learning. In this article, the author talks about formative assessment and its four core elements: (1) identifying the "gap"; (2) feedback; (3) student involvement; and (4) learning progressions. To use formative assessment successfully in the classroom, teachers need specific knowledge and skills. Four basic elements of teacher knowledge are critical: (1) domain knowledge; (2) pedagogical content knowledge; (3) knowledge of students' previous learning; and (4) knowledge of assessment. In addition to an appropriate knowledge base, the successful implementation of formative assessment requires specific teacher skills. Teachers need to be able to: (1) create classroom conditions that allow for successful assessment; (2) teach the students to assess their own learning and the learning of others; (3) interpret the evidence; and (4) match their instruction to the gap. (Contains 15 endnotes.)