This presentation will address the purposes and evolving formats of the case-study approach to research Impact measurement. Using examples from the 2014 UK Research Excellence Framework, it will explore the nature and scope of the claims case studies typically put forward, and the kinds of data that make for more and less persuasive evidence of the impact claimed, its scope, and its significance. Brief attention will be paid to the question of how best to represent collaborative research, and to ongoing debate about how best to secure that quality of research remains the primary goal of research assessment.