The assessment of students is used for various different purposes within an assessment system. It has an impact on students, teaching and the curriculum, the nature of this impact depending upon how it is carried out. In order to evaluate the advantages and disadvantages of particular assessment procedures, criteria need to be applied. This article discusses the criteria of construct validity, reliability, desired impact (consequential validity), and good use of resources, and applies them to assessment for formative and summative purposes. It concludes that for these purposes, the criteria are more readily met when there is greater use of teachers' judgments in assessment rather than external tests.