Today, teachers are expected to develop complex skills, such as research skills, in their students while implementing new views on learning and teaching and using authentic assessment strategies. About these new assessment strategies there is much debate and teachers are vulnerable in using them. We studied upper secondary education natural and social science teachers' practices using two surveys and two rounds of expert panel judgement on teacher-submitted assessment-related material and information. Our study shows that there are grounds for concern regarding the clarity of teachers' assessment criteria, the consistency between teachers' goals, assignments, and criteria, and the validity and acceptability of teachers' assessment practices. The extent to which it is justifiable to judge teachers' assessment practice by professional quality criteria is discussed, and suggestions are given as to the main quality criteria for formative and summative assessment and as to ways in which teachers could improve their assessment practices.