An international idea is that the world must be ‘prepared’ for any disaster situation. Among the many tools and practices that contribute to this frame, the paper focuses on exercises intended to prepare for natural disasters: real-scale simulation exercises. The object of this paper, based on several studies conducted at sites overseen by the UN natural disaster reduction agency (ISDR) and on a field study of a simulation in Peru in November 2010, is these exercises and their purposes and outcomes. It also explores the conditions of possibility for their ethnography.