Teachers draw on an amalgam of knowledge bases in their summative assessment decision making. However, research indicates that teachers' actions do not necessarily reflect their knowledge of good summative assessment practice, as other factors are at play. This article draws on findings from a qualitative study to explore what drives the decision making of beginning teachers as they develop summative assessment literacy. Gess-Newsome's Model of Teacher Professional Knowledge and Skill provided a conceptual framework for the study. This model conceptualises the translation of knowledge to classroom practice as being influenced by a range of amplifiers and filters as it passes through the lens of the teacher. Teachers in this study indicated that contextual and personal factors acted in the role of amplifiers and/or filters as they personalised assessment knowledge and as they made decisions about their assessment actions in classrooms. This sometimes led to surprising outcomes such as teachers purposefully ignoring assessment protocols. This article proposes that teacher cognisance of the presence and impacts of amplifiers and filters on their assessment decision making is valuable.