This article concerns the use of assessment for learning (formative assessment) and assessment of learning (summative assessment), and how one can affect the other in either positive or negative ways. It makes a case for greater use of teachers' judgements in summative assessment, the reasons for this being found in the research that is reviewed in the first sections of the article. This research, concerning the impact of summative assessment, particularly high-stakes testing and examinations, on students' motivation for learning and on teachers and the curriculum, reveals some seriously detrimental effects. Suggestions for changes that would reduce the negative effects include making greater use of teachers' summative assessment. However, this raises other issues, about the reliability and validity of teachers' assessment. Research on ways of improving the dependability of teachers' summative assessment suggests actions that would equally support more effective use of assessment to help learning. The later sections of the article address the issues and opportunities relating to the possibility of assessment that serves both formative and summative purposes, with examples of what this means in practice, leading to the conclusion that the distinction between formative and summative purposes of assessment should be maintained, while assessment systems should be planned and implemented to enable evidence of students' ongoing learning to be used for both purposes.