The teacher assessment that is the subject of this paper is an essentially informal activity. The teacher assesses the student by posing questions, observing activities, and evaluating work in a planned or ad hoc way. The information obtained may be partial or fragmented, but repeating such assessments over time will allow the buildup of a solid and broadly based understanding of student attainment. Such assessment can certainly be formative. Examination of the assessment procedures of teachers preparing the teacher-assessment element of the British National Assessment suggests that teachers may be categorized by their approach to assessment as intuitives, evidence gatherers, and systematic planners. Issues of reliability and validity are discussed, and the moderation of teachers' assessments by the common or consensus judgments of a group or panel of teachers or experts is reviewed. Such moderation is a feature of the British National Assessment. While assessment that is internal to the school is more professionally rewarding to the teacher in terms of enhancing teaching and learning and is more valid, assessment that is to be used outside the school must be more demonstrably comparable across teachers, tasks, and pupils. Statistical moderation and other forms of quality assurance can help ensure this. (Contains 23 references.) (SLD)