"Collective behavior" is defined in the social science literature as "emergent group behavior in settings where cultural guidelines are nonspecific, inadequate, in dispute, or lacking altogether" (Marx and McAdam, 1994:3) and as "those forms of social behavior in which usual conventions cease to guide social action and people collectively transcend, bypass, or subvert established institutional patterns and structures" (Turner and Killian, 1987:3). Using a phase-oriented approach, this paper first discusses collective behavior in the pre-event or warning stage of disasters, then focuses on collective behavior during and following disaster impact. In addressing threat-related and pre-impact social behaviors, collective behavior is discussed under the topics of rumor and information-seeking, warning response, and evacuation decisionmaking. The discussion of collective behavior during and following disaster impact considers social bonds and the myth of panic, disaster convergence into disaster-stricken areas, helping behavior (disaster volunteers and emergent groups), and emergent multi-organizational networks. The overall theme of this paper is that actions undertaken by official agencies and institutions prior to and during crises have a significant influence on collective behavior that subsequently develops. The potential for maladaptive collective behavior in response to crises increases when the public lacks an understanding of the threats it faces, when public risk communications and warnings are vague and inconsistent, when officials create confusion and fear by withholding information the public needs, when the public lacks confidence in its leaders and crisis management institutions, and when the performance of those institutions during crises further undermines that confidence. 81 references