Drawing on her research into students' evidential reasoning, Elisabeth Pickles explores the possibilities for how such reasoning might be assessed. Existing exam mark schemes focus too heavily on generic processes involved in the analysis of source material and insufficiently on the historical validity of reasoning and conclusions produced. Approaching the assessment of sources differently would, Pickles suggests, help students to understand that some conclusions are better, historically, than others. This raises some important and complex questions for teachers and examiners when designing the mark schemes against which students are to be assessed. (Contains 7 figures.)