Assessment issues often get a bad press and yet assessment is at the heart of teaching and learning. Without assessment, teachers' decisions about what pupils need next to make progress would be pure guesswork. They would not know which activities to select and so their attempts to help their pupils would be trial and error. Without assessment, they would be lost when they had to report on pupil progress to important stakeholders such as parents, Ofsted or head teachers. So a good understanding of what assessment can and cannot do is useful for teachers. This article focuses on assessment purposes, and forms the first in a series of three articles on assessment. The authors look at the issues of assessment that teachers face in the classroom.