In the United Kingdom, the majority of national assessments involve human raters. The processes by which raters determine the scores to award are central to the assessment process and affect the extent to which valid inferences can be made from assessment outcomes. Thus, understanding rater cognition has become a growing area of research in the United Kingdom. This study investigated rater cognition in the context of the assessment of school-based project work for high-stakes purposes. Thirteen teachers across three subjects were asked to "think aloud" whilst scoring example projects. Teachers also completed an internal standardization exercise. Nine professional raters across the same three subjects standardized a set of project scores whilst thinking aloud. The behaviors and features attended to were coded. The data provided insights into aspects of rater cognition such as reading strategies, emotional and social influences, evaluations of features of student work (which aligned with scoring criteria), and how overall judgments are reached. The findings can be related to existing theories of judgment. Based on the evidence collected, the cognition of teacher raters did not appear to be substantially different from that of professional raters. (Contains 2 figures.)