If assessment drives learning and the closed book examination dominates the pattern of assessment for undergraduate mathematics (as it does in the UK), lecturers need to ensure that examinations reflect the learning they value. This article uses a mixed method approach to explore lecturers' views of the closed book examination in relation to other assessment methods, their preferences for different types of methods and the extent to which they discriminate between stronger and weaker students. A survey of staff views confirms the dominance of closed book examinations, but an accompanying interview study shows hidden complexity to this view. In particular a tension between the potential and the reality of examinations is uncovered and suggestions are made for resolving that tension.