This discussion paper is produced as a contribution to current debates about the function and form of needs assessment and other diagnostic processes in crisis contexts. The term ‘diagnostics’ is used here to describe the whole range of techniques used to assess the existence and nature of a humanitarian crisis, the appropriate responses to it, and the impact and effectiveness of such responses. This paper forms part a three-year HPG project concerned with the way in which international humanitarian actors understand the contexts in which they intervene and their impact on those situations. Central to this is the link between situational analysis and decision-making about crisis response.