As current procedures for teacher assessment are often based on non-standardized, qualitative information derived from multiple sources, the overall validity of the assessment depends heavily on the judgement processes of the assessors. Because it is of great importance for assessors to be aware of their own judgement processes and of the possible threats to validity in these processes, investigating assessors' perceptions is of vital significance. In the present study, the perceptions of 22 assessors who judged a student teacher pair-wise using a specific assessment procedure were explored using semi-structured interviews. A qualitative analysis of the individual assessors' perceptions with regard to the essential judgement processes of consideration of evidence and combination of evidence to attain an overall judgement resulted in an overview of successful strategies and threats underlying a valid assessment process. General implications for ensuring the validity of the assessment process and the preparation of assessors are discussed.