This book describes how middle school science teachers, in collaboration with a team of researchers, tried to improve their everyday assessment practices to enhance student learning. It describes the challenges they faced, the differences among the teachers, and the personal nature of deep educational change. A product of CAPITAL (Classroom Assessment Project to Improve Teaching and Learning), a research effort supported by the National Science Foundation, this book: uses classroom stories to show how teachers can use a variety of formative assessment techniques to answer questions they have about their teaching; provides real-life examples of teachers grappling with new practices at a personal level, in their own settings and in light of their own values and beliefs; offers suggestions for designing professional development efforts that recognize the significant variation among teachers in how they go about changing their assessment practices; and, outlines principles and practices that must accompany change in the classroom if it is to be more than superficial.