Many teachers say that they do "assessment for learning" (AfL), but often their assessment practice does not really reflect the intentions and principles that make AfL powerful. Teachers who understand the "spirit" as well as the "letter" of AfL are continually building their expertise so that they can carefully apply their professional knowledge on a moment-by-moment basis. They are routinely engaged in seeking, reflecting upon, and responding to information from dialogue, demonstration, and observation, with ideas and feedback that are immediate and directed at learning, in real time. These teachers need policy support, organizational structures, and professional development, so that they can use this knowledge and its application to enhance learning for all students. (Contains 13 endnotes.)