The current spotlight on assessment in education raises ethical issues as practices evolve. This study documents ethical conflicts faced by teachers in the United States regarding assessment of students. Critical incidents generated by practising teachers revealed a majority of reported conflicts related to score pollution, and conflicts frequently arose between teachers' perceptions of institutional demands and the needs of students. The most frequently mentioned assessment topics causing conflict included grading, standardised testing, and special populations. These findings suggest that explicit guidelines for defining and avoiding unethical behaviour would be helpful to teachers in developing their assessment practices. (Contains 4 tables.)