This September 2010 article is a reprint of the original October 1998 article. Few reform initiatives focus on what goes on in the "black box" of the classroom, on what really happens in the interaction between teachers and students. This article is about the inside of the black box. The authors focus on one aspect of teaching: formative assessment. The main plank of their argument is that standards can be raised only by changes that are put into direct effect by teachers and pupils in classrooms. There is a body of firm evidence that formative assessment is an essential component of classroom work and that its development can raise standards of achievement. They consider different ways in which formative assessment can be improved, and then offer four steps to implementing improvement strategies.