This chapter outlines some of the definitions and concepts that lie behind understanding the impact of disasters on the health and welfare of the affected communities.  We first define varied meanings of the term disaster and the (fuzzy) boundaries of research that aims to understand the mental health consequences of these events.  We then describe the traditional typology that has guided this field of study, noting distinctions among natural disasters, technological accidents, and sudden episodes of mass violence.  Next, we describe other important characteristics of disasters and disaster exposure and conclude by elaborating on the temporal dimension of disaster impact and recovery.  Chapter 2 (Norris and Elrod) then delves into the effects of disasters drawn from the research to date.