The process by which an assessor evaluates a piece of student work against a set of marking criteria is somewhat hidden and potentially complex. This judgement process is under-researched, particularly in contexts where teachers (rather than trained examiners) conduct the assessment and in contexts involving extended pieces of work. This paper reports research which explored the judgement processes involved when teachers mark General Certificate of Secondary Education (GCSE) coursework. Thirteen teachers across three subjects were interviewed about aspects of their marking judgements. In addition, 378 teachers across a wider range of subjects completed an associated questionnaire. The data provide insights into the way that criteria are used, the role that comparison plays in the process and the importance of various professional experiences to making assessment judgements. Findings are likely to generalise to "controlled assessments" which have replaced coursework in the GCSE. (Contains 5 figures, 3 tables, and 5 notes.)