The research informing this paper set out to investigate the qualitative experiences of students, supervisors and tutors involved in a summative collaborative assessment of placement learning on an undergraduate professional qualifying programme in the UK. Analysis of data gathered through semi-structured interviews provides valuable insights into its use. In particular, engagement in a collaborative assessment brings a degree of exposure for all concerned. Participants need to feel "able" to participate, and ability to participate is linked to notions of agency and autonomy. Collaborative assessment design needs to acknowledge power relations and include strategies to reduce imbalances. The importance of recognising and working with the complexities of assessment should not be underestimated. Criterion referencing offers a guide to support consistency but can also falsely present assessment as mechanistic and instrumental. The research shows that it is in the "doing" of assessment that a "shared language" is developed. It demonstrates the value of active student engagement in assessment in relation to the construction and use of theory, developing reflective practice and assessment literacy. The collaborative assessment provides students with the opportunity to experience a "performance of understanding" in a community of practice that, whilst pressured, reflects "real" professional life.